[joedoh]

the muffler shop told me that long stretches of tube were what caused the drone, either long pipes to the muffler or long pipes after the muffler. I had a bad drone on a truck with a muffler right before the tip and he put in a small center resonator right in the middle of the long pipe, I didnt lose any exhaust sound but the drone was gone. years later I had a truck with mufflers under the cab and long pipes out the back, it droned enough to make you contemplate driving off a cliff to end it, I replaced the mufflers under the cab with cherry bombs and moved the mufflers back to almost the axle, the drone was gone.

It makes sense on paper, the principle is called standing waves in tubes. take a length of a 2" diameter hose and swing it in a circle, at a certain speed it will howl, and howl at a higher pitch (harmonics) at twice/three/four times the original speed. even if you move your hand so that less of the hose is spinning, the frequency of the howl remains the same because the length of the hose is what causes the standing wave, air bouncing between the ends makes a long standing wave. the way to eliminate the howl is to shorten the hose, so that it takes a lot more energy to spin the hose fast enough and produces a howl at a shorter standing wave (higher frequency). notice that the hose is still 2" in diameter so you havent limited the throughput, just raised the frequency of the standing wave.