My 1998 C-Class (W202) recently started making a strange noise. It is a low, dull, fast chugging sound from the rear of the car. The noise is particularly apparent above 55mph. The tempo of the chugging increases with speed. The noise goes away completely on braking.

Also, when the car is creeping forward on a light footbrake there is a scrape/knock noise from the rear, once every turn of the road wheels.

I took the car to the dealer last night and the tech diagnosed warped rear discs (rotors). Makes sense to me. What do you think?

The car has 47,000 miles. What causes discs to warp?
Can I safely drive the car until it is fitted with new discs next Wednesday, in which time I will do about 200 miles? (The tech said vibrations would start to be felt in the car and I have noticed that this morning).
I take it that I should also have new pads fitted.

As ever, thank you for your comments.

Have your tech "gauge" the rotors to verify that they are worn below minimum or warped. Do make sure you install new pads when you replace the rotors. Go with high quality parts. You may want to get him to change the brake fluid while he is on this job. It should not add too much more to the $ and I have found that it extends the life of the calipers.
Jim
'85 300D
'95 E320
'97 CRV

I'm going to have new pads fitted at the same time as changing the rotors. Looking at the pad thickness through the wheel spokes, it would appear that they are one half to two thirds of the way through their working life anyway.

I'm using MB parts at the MB dealership. I don't know about a brake fluid change as that was done 4 months and 8,000 miles ago so shouldn't really be necessary. The price I've been quoted isn't bad: GB pounds 170, which is about US$250.

I'm now almost certain the rear nearside (left--my car is rhd) disc/rotor is warped: careful listening shows the sound is coming from that area; the sound goes away on braking and there is a faint vibration in the pedal; and when I pulled away from my office this evening, it was as if the rear nearside brake had become bound on, in the way that the brakes on a car with rear drums bind on when the car is left standing for a long period with the parking brake on (I say "like" this--my car obviously has rear discs and I don't use the parking brake when parking on level ground).

Thank you for your comments.