6-cylinders in general, and I-6's in particular will have a few RPM ranges where the exhaust note will resonate and become irritating to anyone who hasn't blown his ears out with rock music, or in my case, jet engines, race cars, and deer rifles. The trick is to position the muffler(s) somewhere in the exhaust stream where the objectionable BRRRRRRAAAAAAAPPPPPPPP noise doesn't happen during normal driving. Moving the muffler to different positions in the exhaust stream is one way, or adding a second small muffler as a resonator, or adding a crossover pipe will change the resonant RPM. The other way is to install a very quiet, low-restriction muffler, but that eliminates almost all the sound. That would be my choice!
Jerry
