Installed a pretty little 4-1 MAC header and muffler. No surprise, it buzzes from 2-5 K rpm. Not a little hum, this was BAD. I ground off the rivet head holding the baffle and used a slide hammer to force the very tight baffle out. First I bent the edges of the inside support ring out to make it fit tighter. No change. Slide hammer it out again, cut grooves in the same support and bent them out to tighten the fit. Still bad. I then drilled 4 quarter inch holes in the support and coated the outside edge with high temp rv. Problem cured. Sounds good, no rattle or buzz.
