mishershmitt

earlier today i was driving my rx8 and i noticed that when i let off the gas pedal there a knocking sound coming from the back of the car. at first i thought it was my exhaust and i was backfiring because i have my cat gutted out but then i realized it would change depending on the speed i was going. say if i was going 50 MPH it would be knocking very fast and as i would slow down it would slow down and stop. it doesn't do this at idle only when moving. does anyone have any idea what this might be?