I need some help with a rather loud rotating noise, but it's difficult to describe. Any help/guess as to what it is would be appreciated.

It occurs at at low speeds when I take my foot off the gas. It sounds like something heavy rotating and grinding a bit. As soon as I step on the gas, there's no more noise.

It's much more noticeable inside the cabin - I don't hear it outside with the windows open. Also, it seems to be coming from the around the transmission area, but I'm not 100% sure where it's coming from just yet.

The car is a 2004 E55 with 150k on it.

Is it wheel speed or engine speed related?
If you put the transmission in "N" does the sound stop?
What service has been done on the transmission recently?
Any work to the rear differential?
How many miles on your current tires?

It seems to be engine speed related. The transmission had a complete flush service about 10k ago. In park, at idle, there's no noise. There's been no diff work that I'm aware of. Tires are in very good shape. The noise seems to only appear in Drive, when I take my foot off the gas. It's very noticeable, though.

I'll take the car out today and try to get more specifics...

when you say rotating, do you mean like a whining sound, almost like a supercharger? The reason I ask is because I had a similar issue when letting go of the gas at low rpms it turned out to be low power steering fluid.

Thanks, but no, I don't think that's it.

I took the car out today and, when cold, it's very loud. I took a video, but it didn't come out great. I'll post it later...

It seems to be coming from the tranny tunnel and is very noticeable inside the car. Above 30mph it goes away. I've confirmed that it's not really related to acceleration vs deceleration. It occurs with both.

Could it maybe be the driveshaft bearing? Relative to the interior of the car, where is that located, maybe under the rear center heater vents?

maybe trans mount needs replacement ?

Could be torque converter going of incorrect tranny fluid level. I'm almost at 150k on mine and I think Ive encountered what you're describing. And there seems to be less stress when the car isn't in drive. No solution yet, I'm going to addressing my issue next week...

I have the same issue since this year in my CL55. Its a grinding sound and i only have it in (what i belive) the first gear... So when driving around on a parking lot with very low throttle and speed and suddenly i put my foot off the pedal, the car decelerates and it makes that grinding sound until my car comes to the speed where it auto crawls.
At one point beginning this month, as the gearbox Oil was still cold, it even made some mechanical metallic sound together with it, that was clearly noticeable from outside and louder than from inside (i had my windows open). I thought like "WTF, is my gearbox now breaking apart?". Haven't had that metallic sound again since that day, but still.

So i also think its clearly gearbox related. However i have still perfect shifts with my car, it shifts just great.

I think i will need to check my front flex-disc just in case as i also developed a rhythmic vibration in the car, whenever i accelerate and its especially noticeable when driving up a steeper hill and accelerating just so to maintain constant speed

Thanks for the responses...

I have no noticeable vibration, and in neutral, I can rev freely with no issues.

I'm starting to think it's the bearing in the torque converter. Does that seem reasonable, has anyone had one fail?

Took the car to the dealer today, they said it's the driveshaft bearing. I'll be picking it up tomorrow, hopefully that's all it was.

I have the same idea with mine, maybe its that bearing...

Center carrier bearing?

That IS the driveshaft (center) bearing.

Was looking for confirmation, thanks

... A quick update. It was definitely the driveshaft bearing. No more noise, and the driveline sounds nice and smooth.

I have a strange sound now as well. It happens in any gear. Under 3,000RPM it sounds like marbles rolling around. Drivability is not an issue. No rough shifting or engagement. I bottomed the car out last night and this morning sounded like it was about to explode. No oil, no visible damage to exhaust. It doesn't do it when it's hot, just cold/warm start.

After reading the post I think the same problem exist on my e350, at 22mph the sound is the loudest. It has taken 40,000 miles for it to get to the point, that the carrier bearing needs replacing. My description of the sound is vary similar to a wheel bearing going bad. In fact I replaced all the wheel bearings thinking they might be the problem. Sound still priest. I will update after replacing the bearing.

Whats the update? I just noticed this sound also. I The carrier bearing looks simple to swap and very cheap bearing. So Im going to do this.

My car started doing the same. No drivability issue just sounds noisy only when decelerating. Its weird then it slows down and goes away if I sit there. Im pretty sure its the carrier bearing you guys mentioned.

Is there anyway to check on the lift?

I replaced the bearing thinking this would be the answer to the noise. The job is not that difficult (Have accesses to a car hoist), I wouldn't attempt the job on car jacks. It took a couple hours and second set of hands at times to help in the removal and the installation of the shaft.
When I got the shaft out the carrier bearing wasn't great but didn't show the signs I was looking for: however the universal joint unfortunately had a what I would describe a flat spot when rotating it. The hole drive line would need replacing, or sent to a drive line outfit as described in this forum.(I feel this a poor design not to be able to replace the universal).
Ok that being said, the sound was noticeable different after the new carrier bearing was installed but came back to the original within a few miles. Rotating the drive line beyond its normal axis must of changed the orientation of the needle bearings in the universal. This is my best guess.
I traded it in for a ML320 CDI problem solved!!!

Do you know how the determined it was that? I see the driveshaft carrier bearing is cheap and looks easy to replace.

I'm not sure how the dealer officially diagnosed it.