To Everybody Above: Thanks very much. The magnet test confirmed the calipers are alloy. I'm close to needing rotors and pads, and will do just that. Since I now have a clear explanation, humming or singing alloy calipers are fine with me, as it gives the car a little more charm, and lightens the front by a pound or so.

that thing happened to my 500sel- what I did is to rebuild my calipers replaced the rubber seals( o-ring0 but not the one in between the caliper. Whats happening is the piston on the caliper is starting to lock-up. So when you apply the brakes it will push the pads but it will not release or suck back.

Wow ... did you notice if the car would pull to one side without the brakes being applied?

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1992 500E (Daily Driver)
2004 Porsche RUF 955 Dakara 550