Having worked out several other problems with my 1976 Cadillac Sedan DeVille
500 cubic inch, an exhaust noise remains under certain conditions--idling in neutral about 800 RPM, while driving under slight accelerator pressure, and while coasting. The sound is usually a mellow roar, except while coasting when it is a coughing rumble (like a defective muffler). At times the noise seems to introduce a vibration into the car which is relieved by shiftinf into neutral. The entire exhaust system has been replaced from the catalytic converter outlet back. There is no sound of leaking anywhere, and the roar at idle is clearly from the muffler area or out the end of the tailpipe. New plugs, rotor, cap, vacuum advance--even replaced the new muffler in case it was internally defective. Timing, idle speed and mixture appear to be correct. The converter heat shield looks bad, but, as above, there is no sound of leaking.I wonder if a loss of catalyst beads would affect the muffler's ability to cancel out noise. Original approx 19 by 12 inch "pancake" converters are not available and rebuilt ones are pretty rough. I question if the new small ones would be satisfactory.
Any thoughts on what might be the thing to do to correct this? TIA.