Ford Mustang (1999-2004) Service Manual: Noise, Vibration and Harshness (NVH) (Description and Operation)

Noise is any undesirable sound, usually unpleasant in nature. Vibration is any motion, shaking or trembling, that can be felt or seen when an object moves back and forth or up and down. Harshness is a ride quality issue where the vehicle's response to the road transmits sharply to the customer.

Harshness normally describes a firmer than usual response from the suspension system. Noise, vibration and harshness (NVH) is a term used to describe these conditions, which result in varying degrees of dissatisfaction. Although, a certain level of NVH caused by road and environmental conditions is normal. This section is designed to aid in the diagnosis, testing and repair of NVH concerns.

Acceptable Noise, Vibration and Harshness

All internal combustion engines and drivelines produce some noise and vibration; operating in a real world environment adds noise that is not subject to control. Vibration isolators, mufflers and dampers reduce these to acceptable levels. A driver who is unfamiliar with a vehicle can think that some sounds are abnormal when actually the sounds are normal for the vehicle type. For example, Traction-Lok differentials produce a slight noise on slow turns after extended highway driving. This is acceptable and has no detrimental effect on the locking axle function. As a technician, it is very important to be familiar with vehicle features and know how they relate to NVH concerns and their diagnosis. For example, if the vehicle has automatic overdrive, it is important to test drive the vehicle both in and out of overdrive mode.

Diagnostic Theory

The shortest route to an accurate diagnosis results from:

  • system knowledge, including comparison with a known good system.
  • system history, including repair history and usage patterns.
  • condition history, especially any relationship to repairs or sudden change.
  • knowledge of possible sources.
  • using a systematic diagnostic method that divides the system into related areas.

The diagnosis and correction of noise, vibration and harshness concerns requires:

  • a road or system test to determine the exact nature of the concern.
  • an analysis of the possible causes.
  • testing to verify the cause.
  • repairing any concerns found.
  • a road test or system test to make sure the concern has been corrected or brought back to within an acceptable range.

Glossary of Terms


An increase in speed at less than half throttle.


An increase in speed at half to nearly full throttle, such as 0-97 km/h (0-60 mph) in approximately 30 seconds.


An increase in speed at one-half to full throttle, such as 0-97 km/h (0-60 mph) in approximately 20 seconds.

Ambient Temperature

The surrounding or prevailing temperature.


The quantity or amount of energy produced by a vibrating component (G force). An extreme vibration has a high amplitude. A mild vibration has a low amplitude.


Gear teeth clearance.


Low frequency or low pitched noise often accompanied by a vibration. Also refer to Drumming.

Bound Up

An overstressed isolation (rubber) mount that transmits vibration/noise instead of absorbing it.

Brakes Applied

When the service brakes are applied with enough force to hold the vehicle against movement with the transmission in gear.


Strong noise fluctuations (less than 1000 Hz) caused by gusting winds. An example would be wind gusts against the side glass.


A low-pitched sound like (200-5000 Hz) that from a bee. Often a metallic or hard plastic humming sound. Also describes a high frequency (200-800 Hz) vibration. Vibration feels similar to an electric razor.


The angle of the wheel in relation to the true vertical as measured looking from the front of the vehicle.

Camber is positive when the wheel angle is offset so that the top of the wheel is positioned away from the vehicle.



The angle of the steering knuckle in relation to the true vertical as measured looking from the side of the vehicle.



Item Description
1 Positive caster
2 True vertical
3 Steering axis


A pronounced series of rapidly repeating rattling or clicking sounds.


A short-duration high-pitched noise associated with a slipping drive belt.


A repetitious low-pitched sound. A loud chuckle is usually described as a knock.


A sharp, brief, non-resonant sound, similar to actuating a ball point pen.


A hydraulic knocking sound. Sound occurs with air pockets in a hydraulic system. Also described as hammering.

Clunk/Driveline Clunk

A heavy or dull, short-duration, low-frequency sound. Occurs mostly on a vehicle that is accelerating or decelerating abruptly. Also described as a thunk.


Releasing the accelerator pedal at cruise, allowing the engine to reduce vehicle speed without applying the brakes.

Coast/Neutral Coast

Placing the transmission range selector in NEUTRAL (N) or depressing the clutch pedal while at cruise.

Constant Velocity (CV) Joint

A joint used to absorb vibrations caused by driving power being transmitted at an angle.

Controlled Rear Suspension Height

The height at which a designated vehicle element must be when driveline angle measurements are made.

Coupling Shaft

The shaft between the transfer case and the front drive axle or, in a two-piece rear driveshaft, the front section.


Cycles per second. Same as hertz (Hz).


A mid-frequency sound, related to squeak. Sound varies with temperature conditions.


A metallic squeak.


Constant speed on level ground; neither accelerating nor decelerating.


The process of a vibrating component going through a complete range of motion and returning to the starting point.


A unit of measurement, referring to sound pressure level, abbreviated dB.

Drive Engine Run-Up (DERU) Test

The operation of the engine through the normal rpm range with the vehicle standing still, the brakes applied and the transmission engaged. This test is used for noise and vibration checks.

Driveline Angles

The differences of alignment between the transmission output shaft, the driveshaft, and the rear axle pinion centerline.

Driveline Angles


The shaft that transmits power to the rear axle input shaft (pinion shaft). In a two-piece driveshaft, it is the rearmost shaft.


All power transmitting components from the engine to the wheels; includes the clutch or torque converter, the transmission, the transfer case, the driveshaft, and the front or rear drive axle.

Drivetrain Damper

A weight attached to the engine, the transmission, the transfer case, or the axle. It is tuned by weight and placement to absorb vibration.


A low frequency (100-200 Hz) steady sound, like a freezer compressor. Also described as a moan.


A cycling, low-frequency (20-100 Hz), rhythmic noise often accompanied by a sensation of pressure on the ear drums. Also described as a low rumble, boom, or rolling thunder.

Dynamic Balance

The equal distribution of weight on each side of the centerline, so that when the wheel and tire assembly spins, there is no tendency for the assembly to move from side-to-side (wobble).

Dynamically unbalanced wheel and tire assemblies can cause wheel shimmy.

Engine Imbalance

A condition in which an engine's center mass is not concentric to the rotation center, causing excessive motion.

Engine Misfire

When combustion in one or more cylinders does not occur or occurs at the wrong time.

Engine Shake

An exaggerated engine movement or vibration that directly increases in frequency as the engine speed increases. It is caused by non-equal distribution of mass in the rotating or reciprocating components.

Flexible Coupling

A flexible joint.


A drive mode on the dividing line between cruise and coast where the throttle setting matches the engine speed with the road speed.


Mid to high (100-2000 Hz) intermittent sound due to air flow. Similar to a flag flapping in the wind.


The rate at which a cycle occurs within a given time.

Gravelly Feel

A grinding or growl in a component, similar to the feel experienced when driving on gravel.


An abrasive sound, similar to using a grinding wheel, or rubbing sand paper against wood.


Steady high frequency (200-800 Hz) noise. Vacuum leak sound.


A steady low frequency tone (50-500 Hz), sounds like blowing over a long neck bottle.


A mid-range frequency (200-800 Hz) noise between drumming and whine. Also described as a hum.


Mid-frequency (200-800 Hz) steady sound, like a small fan motor. Also described as a howl.


Hertz; a frequency measured in cycles per second.


Out of balance; heavier on one side than the other. In a rotating component, imbalance often causes vibration.


Toward the centerline of the vehicle.


The physical quality of sound that relates to the strength of the vibration (measured in decibels). The higher the sound's amplitude, the higher the intensity and vice versa.


To separate the influence of one component to another.


A heavy, loud, repetitious sound, like a knock on the door.


A constant, low-frequency (100-200 Hz) tone. Also described as a hum.

Neutral Engine Run-Up (NERU) Test

The operation of the engine through the normal rpm range with the vehicle standing still and the transmission disengaged. This test is used to identify engine related vibrations.


To return to an unstressed position. Used to describe mounts. Refer to Bound Up.


Away from the centerline of the vehicle.


A short duration, high-frequency sound, which has a slight echo.

Pinion Shaft

The input shaft in a driving axle that is usually a part of the smaller driving or input hypoid gear of a ring and pinion gearset.


The physical quality of sound that relates to its frequency. Pitch increases as frequency increases and vice versa.

Pumping Feel

A slow, pulsing movement.


Radial is in the plane of rotation; lateral is at 90 degrees to the plane of rotation.



Item Description
1 Lateral runout
2 Radial runout


A random and momentary or short duration noise.

Ring Gear

The large, circular, driven gear in a ring and pinion gearset.

Road Test

The operation of the vehicle under conditions intended to produce the concern under investigation.


A medium-frequency vibration. A slightly higher frequency (20 to 50 Hz) than a shake. This type of vibration is usually related to drivetrain components.


Lateral runout means measuring the movement or "wobble" of a wheel or tire at the sidewall. Radial runout means measuring the out-of-round at the tread surface.


Intermittent sound of varying frequency (100-2000 Hz), sounds similar to shuffling through leaves.


A low-frequency vibration (5-20 Hz), usually with visible component movement. Usually relates to tires, wheels, brake drums or brake discs if it is vehicle speed sensitive, or engine if it is engine speed sensitive. Also referred to as a shimmy or wobble.


An abnormal vibration or wobbling, felt as a side-to-side motion of the steering wheel in the driveshaft rotation. Also described as waddle.


A low-frequency vibration that is felt through the steering wheel or seat during light brake application.


A resonance from flat surfaces, such as safety belt webbing or door trim panels.

Slip Yoke/Slip Spline

The driveshaft coupling that allows length changes to occur while the suspension articulates and while the driveshaft rotates.


A high-pitched transient sound, similar to rubbing fingers against a clean window.


A long-duration, high-pitched noise.

Static Balance

The equal distribution of weight around the wheel. Statically unbalanced wheel and tire assemblies can cause a bouncing action called wheel tramp. This condition will eventually cause uneven tire wear.


A light, rhythmic, or intermittent hammering sound, similar to tapping a pencil on a table edge.


A dull beat caused by two items striking together.


A rhythmic tap, similar to a clock noise.

Tip-In Moan

A light moaning noise heard during light vehicle acceleration, usually between 40-100 km/h (25-65 mph).


The acronym for total indicated runout is TIR.

Tire Deflection

The change in tire diameter in the area where the tire contacts the ground.

Tire Flat Spots

A condition commonly caused by letting the vehicle stand while the tires cool off. This condition can be corrected by driving the vehicle until the tires are warm. Also, irregular tire wear patterns in the tire tread resulting from wheel-locked skids.

Tire Force Vibration

A tire vibration caused by variations in the construction of the tire that is noticeable when the tire rotates against the pavement. This condition can be present on perfectly round tires because of variations in the inner tire construction. This condition can occur at wheel rotation frequency or twice rotation frequency.


A noise or vibration that is momentary, a short duration.

Two-Plane Balance

Radial and lateral balance.


Any motion, shaking or trembling, that can be felt or seen when an object moves back and forth or up and down.


A constant, high-pitched noise. Also described as a screech.


High-pitched noise (above 500 Hz) with a very narrow frequency band. Examples of whistle noises are a turbocharger or airflow around an antenna.

Wind Noise

Any noise caused by air movement in, out or around the vehicle.


The acronym for wide open throttle is WOT.

Tools and Techniques

    Electronic Vibration Analyzer (EVA)
    The EVA is a hand-held electronic diagnostic tool which will assist in locating the source of unacceptable vibrations. The vibration sensor can be remotely mounted anywhere in the vehicle for testin ...

    Other materials:

    Roller Followers
    Special Tool(s) Compressor, Valve Spring 303-452 (T93P-6565-AR) Removal and Installation 1. Remove the LH and RH valve cover. For additional information, refer to Valve Cover LH and Valve Cover RH in this section. 2. Position the piston of t ...

    Evaporative Emission Canister Purge Valve
    Removal and Installation 1. WARNING: The evaporative emission system contains fuel vapor and condensed fuel vapor. Although not in large quantities, it still presents the danger of explosion or fire. Disconnect the battery ground cable from the bat ...

    Pinpoint Tests
    NOTE: Reinstall or install new evaporative emission hose clamps removed or damaged during testing procedures. PINPOINT TEST A: DTC P0442 SMALL LEAK IN EVAP SYSTEM Test Step Result / Action to Take NOTE: Condition P0442 DTC set: less than 0.62 ...