vibration in drivetrain
 

I have a 2000 E430 with 50k. I noticed a vibration when reaching speeds of 40 mph (top gear in transmission). This is mostly noticable when cold and seem to go away when warm. I feel it could be the rear-end or drivetrain. Any suggestions?

MILO, start with a close visual inspection of the front and rear flex discs. Check for hairline cracks, shredding or elongation of the rubber at the sleeves. Just finished replacing the rear flex disc on our 1992 300CE which had a similar vibration. You'll likely need thin 41mm and 46mm wrenches to loosen the driveshaft coupling bolt if you replace the flex couplings yourself, but confirm the sizes beforehand.

If the problem goes away after some driving, the tires could be flat-spotting overnight. This is more common than most people think.

Milo, have long have you owned the 2000 E430 and how long have you noticed the vibration? Flat-spotting of the tires in colder weather is a possibility, but you likely would have noticed that long ago. Has anything changed lately with your car? New tires, hit a pothole, etc.

tires are about 8 months old and I did just have them rotated and balanced, but I dont feel that that is the issue. If it were the tires it would be continual after each rotation of tire. If I could discribe it its more like a hummmm-hummmm-hummmm, if that makes sense.