This old 300D has 4 wheel disk brakes and has started making a noise coming from both rear brakes. The noise is a metal grinding sound that varies in amplitude as the wheel turns and as pressure is applied. The tricky part is that the noise only occures if the car is in reverse. We have owned this car for over 20 years now and this is a totally new sound. I have tried switching from semi-metallic to carbon pads, and added brake shims but it did no good. On a side note, the pads are wearing evenly so i do not suspect a problem with the calipers; and the rotors are new. Thanks far any advice - i am really lost on this one!

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'76 300D