I get a low groaning sound as I apply the front brakes on my W115 1976 300D, especially when the car is first driven (the groaning goes away after the car has been driven for 5 miles or so).

The front brake pads are about half gone and the front rotors and rebuilt calipers are about 4 years old. I don't detect any noticeable vibration in the wheel. I recently replaced the rear rotors, calipers, and pads.

Any suggestions?

Robert