BMW Noise – Whine, Speed Dependent -Variable, Diagnose – Differential – Rear End?
This answer is applicable for many BMWs in addition to the one listed below.
car year: 2000
car model: 540i
Hi, my 2000 540i is nearing the 125k mile mark and everything on it is running great. There is one little issue that I can’t for the life of me figure out what is happening. There is an audible whining inside the care when the car is moving. It usually happens after a few miles of driving and it’s more pronounced after longer drives. Initially I thought it was something with the speakers but when I shut off the radio power it still continued. I did the same with the AC and heating fans and it has no effect on it. The only thing that does have any affect on it is the actual speed of the car. And I checked to see if it was RPM related and it’s not…it’s simply a byproduct of the car’s wheel speed. It’s not a leak in the window seals either because it is much to audible at very low speeds to be from air entering the vehicle. Is this something to do with the alternator or something that is still feeding some static or noise through the speakers even after they are shut off? I am thoroughly confused.
From your description, we believe you may be hearing gear or bearing noise from the differential or a wheel bearing. Since we cannot drive the car, we really can’t narrow it down much further.
Can you notice if the sound changes as you transition from minor acceleration to steady state running to deceleration? Try to keep the car at about the same speed (so the sound does not change do to a speed change) and feather the throttle in and out a bit. Does the sound change as you transition from minor acceleration to minor deceleration? If the sound does change in this testing, it is likely from the differential. In this case, change the fluid, install a good synthetic fluid, usch as our Red Line or Lubro-Moly differential fluids. In the case of a potentially noisy differential, we would recommend the Red Line 75w-140 weight fluid. We would also recommend adding the Lubro-Moly Molybdenum Disulfide (MoS2 or Moly) gear additive (mix the Moly paste with the fluid before adding the fluid to the differential and drive the car as soon as possible, for 10 to 20 miles, or more). If the differential is the source of the whine, it will be due to either gear or bearing wear inside the assembly. The thicker fluid and the Moly additive may help to quiet it down and extend the usable life of the assembly.
If the sound does not change with throttle changes (as outlined above), it may be due to a wheel bearing or another less common issue. We would recommend that you have a knowledgeable technician drive the car, if the source has not become obvious through this testing.
Lubro-Moly MoS2 Gear Treatment:
Finally, if the differential is the source of the noise, this means that parts are wearing at an accelerated rate. We would recommend that you also install one of our super magnetic drain plugs, when changing the fluid. This will collect any additional wear particles, instead of allowing them to circulate throughout the assembly.