Trial 1
To clearly determine the nature of the noise, develop a car on the highway at a speed of approximately 20 km / h. Then gradually increase the speed to 90 km / h, listening at the same time to different types of noise and noticing the speed at which they appear and disappear.
Release the accelerator and without braking, reduce the engine speed. During deceleration, the change in noise is monitored, as well as the moment when the noise increases. Usually the noise comes on and off at the same speeds, both when accelerating and when decelerating.
Trial 2
Accelerate the car to 100 km / h, put the gear lever in neutral, turn off the ignition and allow the car to roll freely to a stop. At the same time, the nature of the noise is monitored at various deceleration rates. When turning off the ignition, be careful and careful. Do not turn the key more than necessary to turn off the ignition, as when the key is further turned to the "Parking" anti-theft device may work.
The noise noted during this test, and consistent with that observed in the first test, does not come from the final drive gears, since they cannot make noise in the second test. 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
When the car is stationary and braked, the engine is turned on and, gradually increasing the frequency of rotation of its crankshaft, the resulting noises are compared with those seen in previous tests. Noises similar to the noises that occur during the first test indicate that they do not come from the gearbox, but 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, raise the rear wheels, start the engine and turn on IV gear. In this case, you can make sure that the noises really come from the gearbox, and not from other components, such as suspension or bodywork.