This bulletin applies to BMW M3, M4 and M2 vehicles built up to July 31, 2018. A hard-knocking noise may be heard from the rear axle during load reversals. Or, a humming noise may be heard along with vibration during partial to full engine load in higher gears. The cause may be the rear differential suspension mount.
Confirm that the noise is being produced by the tuning of the differential mount in one of the following testing conditions:
For a knocking noise:
Drive the vehicle for five to 10 minutes to warm up the driveline. Operate the engine in the 3000 to 5500 rpm range. Let off the throttle and then immediately step back onto the accelerator. Perform these test steps in Comfort, Sport and Spor Plus modes.
For a humming/vibration/droning:
Drive the vehicle while in 6th gear. Operate the engine in the range of 2500 to 3100 rpm (or at 4500 to 5500 rpm).
If these tests confirm the condition, replace the differential suspension mount following repair instruction 33 17 005 in the repair manual (replacing the rubber mounts for the rear axle final drive). Replace with P/N 3 17 8 094 690 (polyurethane rubber rear mount).