Muffler-can keep kinda quiet sounds off at rpm

Don't forget that our A-bodies have natural tendency to create a resonance at between 55 and 60 mph that cannot be tuned out; and it's worse with the windows open. Don't believe it? just tow your car in that speedzone with the engine shut off.
If your engine rpm or exhaust system, excites that resonance, there ya go; instant drone. IMO, the fastback bodystyle is a lil better than big-trunk cars.