If you have brake dust shields, they are loose.

Otherwise, check the sliding part of the caliper for movement other than straight in and out. If it moves sideways, you need to replace the pins and bushings.

Also check for broken springs on the brake pads -- the will rattle if that little spring on top isn't there or is not correctly sitting on the sides of the slot.

The other possibility is a loose dust shield behind the rotor.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!