97 S 500 Brake Squeal

[JFawcett]

I have a 1997 S-500 with 38,000 miles on it. The brakes squeal only when in reverse....guessing there is something in the pads that is making it do that? They don't squeal when you step hard on them, but do when you are slowly backing out the garage or a parking space.
Any ideas on this? Brakes work great and are silent when going forward.
Not a problem, but a Mercedes is supposed to be seen and not heard.....unless you want it to be!