I have a '86 300 E and I have the same problem. I had the rotors, calipers, and brake pads replaced and still the freakin' noise. However, it only occurs after it's been warmed up and I start braking to a stop or slowly release the brake to a crawl and a loud groaning sound that only disipates when I pick up the speed. I still haven't found the noise yet and it's frustrating.