sounds like a brake rotor as suggested earlier...or even a loose pad spring...this can cause the pad to come loose at speeds and create drag and vibration....chased a similar problem in and out of shops for 2 years on a 190E...also watch your piston alignment in the callipers..they have a small step, and must be mounted at the correct position...they can be mounted crooked ..at least in my 2.6 and other 190's this was true..this will cause an uneven wear on the brake shoe and can also lead to minute brake drag..and subsequent vibration/drag