Rear Noise when braking

Our 2005 Ford Taurus makes a rhythmic clunk from the rear when you slow down. The rhythm seems to slow as you brake. The noise isn’t there when you are cruising or accelerating, only when you slow down. Can anyone give a possible explanation? We recently drove 2000 miles with a small trailer attached. Could that have anything to do with it?

Are the rear brakes disc or drum? Do you have ABS?