Need help with exhaust noise on 76 Cadillac

Hello. I've had my 76 Cadillac Sedan DeVille 500 ci since new and it's been well cared for. In recent years it's developed an exhaust noise no one has been
able to figure out.
When idling in neutral, running up the engine to about 800 RPM produces a deep roar. The same sound occurs while driving at a steady speed with just moderate pressure on the accelerator. When taking your foot off the accelerator while moving, there is a sputtering roar (just like a bad muffler sounds). Also, when stopped in neutral, revving up the engine and quickly taking your foot off the pedal produces the sputtering roar.
The exhaust system (all Walker parts) is new from the outlet of the converter back. The sound comes from the muffler (and out the end of the tailpipe), checked with stethoscope to be certain. There is no indication of leakage anywhere in the system. We replaced the new muffler just to be certain there was nothing internal in it causing the problem. The catalytic converter is the original one and the heat shield has some rust holes in it, but no sound coming from it. New plugs, rotor, cap, vacuum advance, vacuum hoses, timing carefully set (6 BTDC at less than 600 RPM). The original type converter is not available and I question if the small replacement ones would be satisfactory. I can get a rebuilt one, but haven't been able to find one without holes in it.
Any thoughts on dealing with this very annoying situation?
TIA
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On 20 Jul 2003 11:52:52 GMT, snipped-for-privacy@aol.com (Halterb) wrote:

While you may have a cat problem (or not, depending on the jurisdiction you live in WRT emissions and older cars), I have one thing you may want to think about: Your air filter cover.
I noticed with my car (and my half-brain as well) that when I forget to replace the air filter cover, I get a god-awful racket whenever the secondaries open up. This may or may not be the problem, but it is a thought :)
Vuarra
Quid quid latine dictum sit altum videtur. (That which is said in Latin sounds profound.)
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Could be an EGR valve, Exhaust heat riser, or cracked manifold. I use a wet rag stuffed up tail pipe, which diverts exhaust to leak. Welded system or clamps ? Clamps don't always seal well.
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