is at operating temp, and only when in Drive or in Reverse. Thinking it could be drive shaft bearing, tensioner, or a belt? Any help appreciated!
This is a 00 ML320 with 40K miles on it.

I've never had your problem, but it could be the Harmonic Balancer, tensioner or the belt or a combination of these parts. On my 00 ML430 they replaced the tensioner & belt when it was in for replacement of the problem Harmonic Balancer. This was done under warranty before the Balancer failed. If you haven't already read about the Harmonic Balancer problem check out this thread Crankshaft Balancer Failure! UPDATED

Power steering pump?

And yes I have read all the posts about the Harmonic Balancer, and am planning on taking care of that right away!

In Park: Very faint at idle, when I apply gas, it stops.
In Reverse: Very noticeable, stops when I apply gas and begin to move.
In Neutral: Very faint at idle, stops when I rev it up.
In Drive: Very noticable, stops when I apply gas and begin moving, at all speeds from 5-50 no noise. Coasting with no gas to a stop while in drive, it is noticable.
Does this help?

I have heard of problems with a driveshaft bearing problem. More so in areas where they are exposed to a lot of road salt.

J-bird...... here's part of a post from the M-Class news group that might help.

http://www.escribe.com/automotive/european/mbmc/index.html

I would guess that the tow truck driver was right in suspecting a drive
axle issue. A number of owners on the list have experienced center
support bearing failure - though I haven't heard of any failures as abrupt
as yours. Most commonly, people in areas with harsh environments (salted
roads etc) report a very noticeable vibration. If you're not familiar
with a center support bearing, it is located halfway along the rear
driveshaft between the transfer case output and rear differential input -
it supports a long driveshaft and enables a change in angle. My guess
could be wrong, but I wouldn't be surprised to hear that this bearing
failed catastrophically on your vehicle. If this is the case, the repair
should not be too terribly expensive - perhaps somewhere in the $200 to
$400 range.

j-bird,
Driveshaft/center bearings do not make noise when vehicle is stopped. Rotating members do, tensioners, p.s. pumps/fluid level,water pumps, torque converters. Belt/ tensioners are a good place to look. While sitting in drive/reverse have a compatriot spray some water at the belt and or tensioner while listening for a change.

__________________
Paul