I have heard exhaust leaks that sound like knocks, they usually get louder under power and seldom audible when idling.
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06 48RE megacab, not cummings equipped but Cummins powered. Edge Juice/Attitude, mod'd intank fuel pump, Fleetguard FS20000 fuel filter and external lift pump regulator, modified intake housing, Goerend tranny/billet input, GDP intake horn.