Low Frequency Vibration Sound
 

I have a 2006 3.0 M54. When sitting at idle at a light or elsewhere with the car in gear in makes a low frequency vibrating noise that is annoying as hell. I've cranked up the idle speed as high as it will allow and it reduces it but doesn't eliminate it. It is more of a sound than an actual vibration that you can feel. When idling in park it doesn't make the noise. It's driving me nuts. I even put the X on jack stands, started it up, put a brick on the brake and put it in gear to see if I could isolate the noise. No luck beyond it seems like it is louder at the rear of the engine, but I couldn't tell anything more specific than that. Anybody else have the same thing? Were you able to find the cause? Any ideas? Car has 292,000 miles on it.

There lots of posts about motor mounts being the issue for that. I imagine worse with A/C on ? headlights?

Try slowing the idle down as much as possible.

The most common causes in no particular order:

CV Axles have slop (very rare)

Transmission mount

Engine mount

Rear diff mount

Giubo

Basically anything in the drive train that can act like a spring to lash back between power strokes.

You can use a vibration measuring app to determine where is the highest amplitude

https://apps.apple.com/app/id1089589547

Here's the one i use.


–awr–

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Definitely worse with the AC on.

I've replaced all of those. The sound continues. Started with motor mounts first then worked my way through the rest as each thing didn't fix the problem. Any idea if the torque converter could cause something like that?

I have that app, but forgot about it. I'll give it a try and see what it says.

Have you tried slowing the idle ?? Going lower than a resonant frequency can be more effective than higher. Did you use OE motor mounts ?? I think we had some posts saying that the problem was even made worse by non OE mounts ??
How long has this been a problem? Have you tried running without belts??

I haven't tried slowing the idle. The noise is reduced by raising it. I'll see what happens by lowering, but I don't think that will help.

The mounts weren't genuine BMW, but FCP said they were OE.

It has been a problem for a couple years now and seems to be getting worse.

I haven't tried running without belts. It's getting cooler now, so I can give that a shot and see what happens.

Is your exhaust missing its resonator?

If not pull it off it should raise the resonant frequency.

It’s located just past the cats.

https://uploads.tapatalk-cdn.com/202...58aaff707f.jpg
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You can add or subtract weight from this to change the resonance frequency so it doesn’t match your idle.

I’m doing the exact dang thing with my n55 70 today. I didn’t put the brackets back on that hold the cat tight to the transmission bell and it shakes the whole car. I’m going to put some weight on to lower the resonance before I replace the brackets so it won’t try to vibrate so much at idle.

Wow they were tuning resonance in our trucks? I want to look into this when I get past the mechanicals/electricals and get to the NVH stuff.

I'll give it a shot. Thanks Andrew