Rumble Sound
 

I've been to a couple events, and have heard some cummins go by that sound almost like a gasser hitting a like with a big cam. At idle, it's like tapping the pedal. Can some one tell me how or what it is that sets the cummins to sound like this.

more then likely heavily fueled trucks with over sized fuel lines

5 Inch exhaust (or stacks) and a lot of timing will make it sound funny.

heavy duty clutches grabb weird and make a clacking sound while idling and quick grabbs.