Spontaneous Low Hum From Rear End

[BrooklynE46]

Hi All,

I recently acquired a 2001 330i from the original owner. It has 130k, but it sat in a garage for years and it is in excellent shape. This forum has been invaluable in learning about the quirks of these vehicles, but there is one issue that I can't put my finger on.

When driving at relatively low speeds from 10-30mph, there is a spontaneous low vibration that seems to originate from the rear. The sound is resonant and of low frequency, but I can't replicate it at will. It does not increase with speed, but instead disappears completely on the highway. At times, it sounds like a subwoofer in the trunk that has an electrical short (I've checked the speakers and they are fine - no sub in there either!) It is more noticeable when the trunk is empty - we drove 300 miles with a completely full trunk last week and it seemed to disappear or was muffled.

There is also some light vibration around 2500-3000rpm below the center console, but this *may* be unrelated. The motor mounts were replaced last year, but I don't think that the Guibo/CSB/transmission mounts have ever been changed. That's on the list to check out. I assume that the rear differential fluid should be changed as well. I'm 99% sure that that is not related to the tires. The wheel bearings also seem to be fine. It does not appear to be related to the suspension as pot holes and other bumps to not cause the sound.

Any other suggestions for tracking this down? CV joint or bad rear axle?I have searched the forums and can't find anyone with this noise.

Thanks in advance for any advice!

[BrooklynE46]

Hi All,

I recently acquired a 2001 BMW 330i from the original owner. It lived in a garage its whole life and has 130k miles. The owner took it for regular service checkups at BMW Manhattan and shared all of the records with me. It is in EXCELLENT shape, but there are a few sounds that I am hoping to track down and have repaired. This forum has been invaluable in learning about these cars and I truly appreciate all of the great advice!


When driving, I have noticed a low frequency vibration that appears spontaneously (or so it seems) around 10-30mph. It is a short, intermittent sound, with a low frequency resonance to it. The trunk is empty and the battery is strapped down tight. I can’t replicate this sound at will and it does not appear to be replaced to the tires or wheel bearings. On the highway, it disappears completely. It sounds similar to a mid-range sub with an electrical short, but I have checked the speakers and they are fine (no sub in the trunk either!).


It’s almost unnoticeable with the stereo on, but I know that something isn’t right back there. Any tips? Bad cv joint or rear axle? The wheel bearings seem fine and I am 99% sure that this in not related to the times, which were all replaced last year. I want to change the rear differential fluid soon as it can’t hurt anything. It seems unrelated to the suspension and rougher roads don’t seem to impact the sound. It appears just as regularly on new pavement.


There is some light drivetrain vibration between 2000-3000rpm when accelerating slowly (this disappears or can’t be detected if accelerating quickly), but I assume that this *may* be unrelated. I don’t believe that the giubo, CSB, or transmission mounts have ever been replaced, so that’s on my to do list if in the near future. The motor mounts were replaced last year. Nonetheless, it’s not clear if this vibration is related to the spontaneous low hum or baaaahhhmmm sound from the rear.




I would love to get 200k out of this car as it’s a pleasure to drive, but I also don’t want to start replacing parts unnecessarily. Any advice would be greatly appreciated!