[joedoh]

got a shop vac? take the hose off and swing about 36" of it (just hold the rest) around in a circle. at a certain speed you will hear a clear note coming from the hose. if you spin it faster the note will change in pitch to the next higher octave. if you cut the hose shorter and spin it at the first speed, the note will change to be slightly higher in pitch than the first note.

what you have just done is demonstrate a "standing wave". big diameter tube with long unbroken stretches are lousy with them. what is happening is the air is bouncing back and forth in the tube at a specific frequency known as the resonant frequency. a resonant frequency is when the input power is the least when compared to the output sound, the easiest frequency it can make.

when that resonant frequency is in the 120-300hz range, it makes an awful drone in the cab. I have had it. in my experience putting the mufflers closer to the manifolds helps with drone (shorter stretches of pipe, less length, higher resonant frequency). if your mufflers are at the back, consider adding a mid-pipe silencer, it can be fancy like the center muffler from a cummins dodge truck (what I use) or it can be as simple as the smallest glasspack/cherry bomb they sell (I saw an 8 inch one), it wont cut down the sound you love, it just breaks that long standing wave.