From the time I bought my '98 C230, there has been an odd sound when the car is moving. If the wheels are turning, the noise is present all the way through the speed range with the pitch changing a little higher as car speed increases. At first, because it sounds sorta like a rumble, I thought it was bearings, it is not. Then I thought it was wheel/tire related, it is not. It is NOT engine speed related, or transmission speed related. I raised the car and got under to isolate the sound, and it's coming from the transmission area, seemingly directly below where the shifter enters the cabin, but that could just be because there is (obviously) a hole there. It is not coming from the driveshaft, and sounds like further into the tranny area. It is too crowded there with exhaust and shields to get any clear access for better diagnosis.

Yesterday, I noticed another possible clue. The AM radio reception has always been very noisy and I can only listen to local stations. While fiddling around, I discovered the whine in AM is directly related to car speed (not engine speed).

The car runs well - no service light(s), no problems at all. Now I'm wondering if there is a rotating sensor on the transmission that could be the culprit? Wax up those crystal balls!