I, too, have a similar problem with my 92 500sel.

Brakes seem to work fine EXCEPT a metal to metal grinding noise (just like worn brakes) from 5mph or less ONLY. No grinding, pulsing, or noise anywhere else.

The brakes were done (just before I bought the car 10k miles ago), and the pads appear to have plenty of meat and there are no scores are other apparent wear to the rotors...

-David