amgqmp1

I recently acquired a '97 E320 Sedan. It has a minor issue that I wanted to get the experts advice on...and you folks are experts. I had the car into the local stealership for this on Wednesday, however, they couldn't replicate it (didn't follow my instructions to either). So...here's my story:

The car makes a high-pitched whining sound between 1500-2000 RPMs -- only when driving. This does not occur with the car in park or neutral. My temporary workaround is to give it more gas (squelches the noise as RPMs go beyond 2K).

If I drive the car normally, it happens frequently (and it's fairly loud). If I accelerate very slowly through the low RPMs, it doesn't seem to occur very often. It sounds like belt whine, disc brake whine, or ???. It doesn't cause any drivability issues...other than annoying me to no end. Nothing like driving a relatively nice car that sounds like an old Yugo. ;-)

Here's what I know of this car mechanically:

1997 E320 Sedan
117K miles
Quality, new tires all the way around
Brakes still feel fine (some minor vibration when decel. off highway exit ramps, otherwise none)
Serpentine belt is an MB part and I assume original (it passes visible inspection by myself and the stealership, tension is proper)
New, proper Bosch plugs (others were original, thus, I suspect many other items to be original...like the serpentine belt)
New air filter
I've inspected the air duct work under the hood and can't find any air leaks or damage...it's like new
The "lifetime" (of a cat or dog?) transmission fluid is presumed to still be original.

Again, this only happens while driving. My best guess is that it's transmission related, although, it's not specific to any particular gears. The car doesn't leak any fluids and the transmission has no signs of prior leakage.

Any thoughts on where to start with this one?