Sounds like exact scenario I had with original motor mounts failing. In park and at speed the vibration and noise would smooth out. So if not the motor mounts, I'd still think its something to do with the engine vibration.

Motor mounts were the first thing I replaced. Didn't help.

Maybe an engine misfire ? No codes ? I would look for that thread were I think someone else went through this, maybe even with the Corteco mounts which FCP claims to be OE. I think the trans mount can be an issue too.

Included in the master list. (Trans mount).

Get under the car with it on stands with engine running you can use a good glove so you can grab exhaust parts and feel around to find the source.

In my case it's the cat. Shakes enough i can see it! It vibrates enough to shake the motor and trans.

I learned the hard way there's a good reason the cat is held down so firmly.

The trans bushing absorbs the shake and the steel mount is stable no vibration.

On the 3.0 E53 the vibration damper is exactly where my vibration source is so check if there's any firm mounts to the cat meant to hold it from vibrating and of course try without the vibration damper to see if the modified resonance is incompatible with your idle.


–awr–

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No misfires and no codes. I replaced the trans mount bushing, the center drive shaft bearing and the diff bushings. None of it helped.

I'm going to try pulling the exhaust vibration damper and see what happens.

I would def start there but also feel with your gloved hand.

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

I use this app for vibration measuring.

It will give you the vibration frequency which can determine if it's something that happens once per 720° (one cylinder power stroke issue) or 120° (every power stroke).

You can also hit a suspect like the cat and measure the Hz etc.


–awr–

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I have that app. I'm going to see what it says too.

Didn't you use it to find wheel/3 frequency = inner CV?


–awr–

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As a matter of fact, yes. With your help!