[iainmd]

Almost guaranteed to be the brake pads rattling in the caliper. I have had this problem myself. Early S40/V50 rear pads had a spring on the inner pad that clipped over a groove in the piston to stop the rattling. Later pads had a rubbery shim pad between the caliper and the piston on the inner pad instead.

I fitted a set of EBC pads that had neither and they rattled almost constantly and clanked when I went over speed humps. I tried copperslipping them to death with no joy and after consulting my trusted local garage they were replaced with Volvo OEM (ATE) pads. Result... rattle gone. I never used anything other than Volvo OEM pads on the rear again until I got rid of the car about 8yrs later.