"Normal" AT sounds 300D Turbo

[xdjio]

Hi All,

I've searched for past posts on this topic but have been coming up somewhat short. Any advice is appreciated!

I've noticed that sometimes (not reliably) I hear an unusual sound that is hard to localize when I am in D and am at a stop. It sounds like a deep, constant thrum, sometimes increasing and decreasing in pitch slowly, almost like a sort of "surging" noise. The engine rpm's are rock solid. But mostly it's a low-frequency thrumming kind of noise.

I hear this noise most when the motor is cold, but have heard it after driving with the car up to normal temperature too. It doesn't happen every time I am in D and stopped, I'd say about half the time.

As soon as I take my foot off the brake and start moving forward (even just slightly) the noise vanishes. Likewise if I shift into N at a stop - the noise goes away then too. I never hear this noise while the car is moving - only when the transmission is engaged and I've got the brakes applied. If I'm in R and on the brakes, I can hear it too. I'm not sure if I can tell if it has anything to do with the car being on level ground, or pointing uphill/downhill.

I just checked the level and condition of my AT fluid and it looks great - nice bright pinkish red, no burned smell, just between the top and bottom level marks on the dipstick.

Any ideas on what could be going on here? I'm pretty stumped. Thanks for any pointers!

Also for reference, I checked the ATF level by doing the following:

1) Drove the car for ~ 45 minutes (with the engine at normal temp, 85C)
2) Parked, left the car off for about 5 minutes or so
3) Started the car, with parking brake on and cycled through the entire shifter range
4) Checked fluid level while engine at operating temperature and idling

I have not checked the fluid level with the engine totally cold. Should I? I think based on all this I am pretty sure the noise is tranny related, but who knows? Maybe torque converter? Maybe something else entirely? Could the noise I am hearing just be "normal" pump noises? I really only know enough theory about automatic transmission to be dangerous

[xdjio]

No ideas? I will be driving in to work shortly, so I will try and pay more attention to the conditions and character of the noise and see if I can be more descriptive...

[rrgrassi]

What shape are the rubber mounts in?

[xdjio]

Right now, they are in totally unknown shape. They could be good, marginal, or dead. I'll dive underneath later this evening and have a look. I'd go now, but my coworkers probably don't want me tracking dirt and road grime all over the office!

[xdjio]

More information: I really only can easily discern the noise when the car is cold after sitting all night. By this I mean - there is an obvious difference in sounds I can hear when I am running the engine in N, and running it in D with my foot on the brake at a stop. But insofar as hearing a loud, pulsating hum, well, that seem to happen only during the first minute or so after starting from stone cold.

I have a theory - maybe it is reasonable? Perhaps this really is as simple as the car generating different vibrational modes when in D and the T/C is stalling due to the car not moving, and I am just hearing that combined with the car being a bit rough due to low temperatures. I know i have a persistent rattle coming from the air cleaner mount (missing a bolt) and if something that small can be so audible, then maybe a worn transmission mount could cause this?

Certainly the car drives well and shifts well - and I ain't no transmission guru but I would think that a serious problem would manifest itself while the car is moving, too...