if only happens when braking almost always the brake pads. I've re-done all 8 brake pad sets on a pair of client cars that sounded like a bus coming to a stop. take off the pads, put some anti-squeal (it's like thread lock) between caliper and brake pad and reassemble. 'I put that shit on everything'. They say not to use with shimmed pads but if they squeal i use it and it's always worked. There is no magic spray for rotor that actually works.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)