I know this is long shot, but anyone have any ideas?
 

Hello,

My 1997 C280 with 87k miles and the 722.6 tranny has started exhibiting a strange noise. It makes a very high pitched whinning noise that only appears at 52-54 mph. It started about a week ago. It sounds like it is coming from the engine/tranny area, but it does this independent of engine rpm. i.e. I change change gears and thus revs and the noise is still there. So the noise does not depend on engine rpm, only road speed.

Also, it only does this when the transmission is operating and relatively cool temps but after it has warmed up a bit, around 100-130 degrees F or so. So, my first inclination is to believe that it is somehow a tranny related issue, but I really don't know. If I sit in traffic for a while so the tranny warms up a bit more, and then try to demonstrate the noise again, the it disappears completely.

The noise sounds somewhat like the noise in this post that Donnie said was the cold oil rushig through the oil pump in the engine. However, on my car, the noise is much higher pitched and does not increase in frequency as the engine revs higher. Also, engine coolant and oil are at operating temp when this noise occurs.
http://www.peachparts.com/shopforum/tech-help/35065-c43-other-w202s-what-exactly-startup-whirring-noise.html?highlight=oil+pump

I understand the extreme difficulties in diagnosing the description of a sound over the internet, but I thought maybe some else may have encountered this specific problem and I might get lucky. Everything works perfectly, but the car did not do this a week ago, it is not normal and I just want to make sure that I do not have any major problem in the future because i neglected some warning signs.

Thank you,
Ali Al-Chalabi

Ali:

How are your tires? I had a whining noise coming from the trans/engine area at 45-50 mph on my C230 (1998) which the dealer said was 'normal' - it almost completely disapeared the very day I installed new tires. There was plenty of tread on the old Goodyears - I switched them to go to 16" rims. Could tires make a noise like that only when warmed up?

JCE, thanks for the reply,

The tires are Michelin MXV4s which have 20k miles and them and look brand new. Pressure is good, and I do not suspect that it is the tires, the noise is only in a certain temp range, as in if it is in the 60s Ferenheit outside, there will be no noise, it has to be relatively cold outside. The reason I stated the trans temp as a possible culprit is that independent of everything else, the transmissionn always seems to be at the same temp when it occurs, but this could be just a coincidence and I could be way off base.