Originally posted by Thom Pintello
There's no other way to describe it. When the brakes get warmed up there is a groaning sound when they're applied. Could the previous owner have put in non OEM pads? I had a similar problem when replacing the front pads on my girlfriend's BMW 733i, when I changed them with non OEM pads there was a groaning sound. Is there a difference in density of these pads that can cause this? Must they be matched carefully with the brake system in question? Thanks in advance for your advice.
Brake noise is caused by vibration somewhere in the components that produce an audible noise. The first thing you should do is a basic visual inspection to verify that the pads are serviceble with even wear. If you are experiencing a "groan" sound with brake application, my first suspicion would be a completely worn down friction material with the pad backing plate bearing against the rotor.
Squeak and squeal noises can be difficult to run down, but sometimes hard brake usage, such as running down a canyon or mountain hard with lots of braking into the corners can cure noise problems, at least temporarily.
Noise is also a function of pad material, and low noise pads such as kevlar or ceramic are often quieter, but not as effective as semi-metallic compounds.
Failing wheel bearings usually make a continuous growling sound that is independent of power or brake application, so if this noise only occurs when the brakes are applied, it's very unlikely that it is wheel bearing related.
The OEM pads on my '88 190E 2.6 worked very well, but they dusted and squealed terribly and were tough on the rotors. I elected to install a set of Repco Deluxe (not Metalmasters) pads at 15K miles based on some advice from other owners, but knew they would be less effective. The car now has 72K miles, and there is plenty of life left in the pads. In recent years the pads seemed to become less effective - probably due to a lot of in-town driving, so I decided to do an "Italian tuneup". On a recent trip to Northern CA I drove the car hard over back roads (State Routes 33, 198 and 25), which required constant braking approaching corners. As a result of the hard use the glaze on the pads was broken and the pads became much more effective, so I am now, again, happy with the brake performance