Strange very low pitch vibration

[NashvillePaul]

My car is a 2004 13,000 mile 330i sedan with a strange noise my dealership can't figure out. I think it may be transmission related but would appreciate anyone's opinion in the hope that it is a known problem. I will try and describe the symptoms:-

It only occurs when the engine revs are about 1200/1300 rpm, above or below that the noise will go. The noise could be described as what you would expect to hear when driving over, say a metal grate in the road, ie a low pitched rumble. You can actually feel as well as hear it! It is driving me mad and is getting slowly worse. Has anyone ever had anything like this? Comments greatfully received..

 

[William330]

Paul, not sure if this is your symptom, but it may be worth a look:

http://www.bmwboard.com/forum/showthread.php?t=21940

 

[NashvillePaul]

Paul, not sure if this is your symptom, but it may be worth a look:

http://www.bmwboard.com/forum/showthread.php?t=21940

Thanks for the link, but no, definately not that. It's a very low bassy sound which sounds like it's under the car although it is difficult to tell as it resonates throughout.

 

[Kirby]

Does this occur when:

- only in park?
- only when driving?
- both?

 

[NashvillePaul]

Does this occur when:

- only in park?
- only when driving?
- both?

Only driving, anything from 10 mph to 40 mph but if the revs hit 12-13K the noise happens. So obviously when doing 40 you need almost no gas so the revs drop. The slower the more aparent, probably because the general background noise drops. I'm thinking toward transmission but I think anyone else who has had the same problem will know what I am describing. That's what I'm hoping for, someone who has had the same thing.

 

[Kirby]

Geez, without hearing it, it could be alot of possible things, but clearly it's a "resonant" vibration because it only shows up at a single RPM band, not speed dependent.

I'm going to think aloud, so to speak:
- probably not the engine or flywheel - if it were, it would do it in park
- probably not the drive shaft/guibo - if it were, it should do at certain speeds, not RPM.

So I guess I agree that it is probably transmission related. Is it manual or auto?

Besides an internal transmission problem, other things could allow the vibration to occur:
- broken engine or transmission mount
- bent or loose frame cross member
- exhaust system hitting something

I had a similar problem on my Suburban. I had a rattle noise only at idle. It turned out to be a bad exhaust bracket. When the engine twisted slightly while in gear, it cause a vibration, in my case only at idle.

 

[NashvillePaul]

It's an auto by the way...

You know, exhaust is a distinct possibility although the dealership say it is fine. But I would have thought it would make a more metalic noise rather than the deep resonant throbing type noise which is RPM related. Of course the exhaust would be RPM related too if it is resonance, but I just don't know. It's a mystery that's for sure.