620 automatic clunk noise on gear shifts

I own a 1994 620 automatic -wonderful trouble-free car- with 64k kilometres. Sometimes I hear a soft "clunk" noise in the middle of the car, but not always, when changing gears at low revs. It seems to do it more clearly when the gas tank is empty. Rover (Spain) does an annual check to the car but hey cannot hear the noise at that moment. Can you help me? Ricardo