Definition of Noise, Vibration, and Harshness

Noise is any unwanted sound, usually unpleasant in nature. Possible sources of noise are:
- Engine.
- Intake and exhaust system.
- Driveline.
- Wind.
- Road surface.

Vibration is unwanted low to medium frequency movement which may be either regular or random in nature. It may be felt or heard. Possible sources of vibration are the:
- Tires.
- Engine.
- Engine accessories.
- Suspension.
- Driveline.

Harshness refers to the ride quality and is very subjective. Reference to previous conditions is usually the only way to identify harshness as a symptom.

All internal combustion engines and drivelines produce some noise and vibration. Mufflers and dampers are used to reduce these to acceptable levels. Four-wheel drive vehicles and vehicles with Traction-Lok differentials produce noise when driven on hard surfaces that would not be heard in two-wheel drive vehicles or vehicles driven on surfaces where wheel slip can occur. If the vehicle has automatic overdrive, it is important to test drive it in both modes. If the driver is unfamiliar with this type of vehicle, the sounds may seem abnormal, but they are actually inherent in this type of vehicle.

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 probable causes.
- Using a systematic diagnostic method that divides the system into related areas.

The diagnosis and correction of noise, vibration, and harshness problems requires:
- A road test to determine the exact nature of the problem.
- An analysis of possible causes.
- Testing to localize the cause.
- Correcting any problems found.
- A road test to make sure the problem has been corrected.