knocking noise when you let off throttle

[COWBOYBLADE]

predator prt # 182491,revision v.1r16, 2006, dodge pickup. just bought predator an installed it love the performance. but have a knocking noise when it idles down. only does this when u let off the throttle.does not do it all the time though.i notice more when it in park an you iddle it up an let off. it just does it when u let off the throttle. need help love the 5.9 cummins with the predator.it does not do this when in stock.