Trial 1
To clearly identify the nature of the noise, drive along the highway at a speed of approximately 20 km/h. Then gradually increase the speed up to 90 km/h, listening at the same time for different types of noise and noticing the speed at which they appear and disappear.
Release the throttle pedal and, without braking, dampen the engine speed.
During deceleration, watch the noise change, as well as the moment when the noise increases. Normally, the noise comes and goes at the same speeds both when accelerating and when decelerating.
Trial 2
Accelerate the vehicle to approximately 100 km/h, put the gearshift lever in neutral, turn off the ignition and allow the vehicle to roll freely to a stop; watch the noise pattern at different deceleration rates.
Warning: When turning off the ignition, be careful and careful. Do not turn the key more than necessary. This may cause the anti-theft device to operate.
The noise noticed during the tests and corresponding to that noticed during the first test does not come from the final drive gears, since they cannot make noise without load.
Conversely, noise noted on the first test and not repeated on the second may come from the gears of the gearbox or the bearings of the pinion or differential.
Trial 3
With the car stationary and braked, turn on the engine and, gradually increasing its speed, compare the noises that have arisen with those seen in previous tests. Noises found to be similar to Test 1 will indicate that they are not gearbox noise and are caused by other components.
Trial 4
Noises found during the first test and not repeated during subsequent tests come from the gearbox; to confirm, lift the rear wheels, start the engine and shift into fourth gear. In this case, you can make sure that the noise really comes from the gearbox, and not from other components, such as suspension or body.