Mondeo knocking noise?

I have a mondeo 1995 m reg manual gear box. When im driving i get a knocking noise from the rear of the car which quickens as the car speeds up. This noise stops dead as soon as the brakes are applied (no matter how hard they are applied or how fast im going). The brakes are at the front of the car tho. Also it still makes the noise if im driving in a straight line. Any advice would be appreciated, thanks.