Do you notice being down on power at all?
I am not sure if this is your problem, but I will share what happened to my 86 that was making a similar noise. The left rear cat has somehow broken off a piece of the honeycomb inside it and this resulted in a metallic pinging as well as greatly reduced performance due to the restriction. I dropped the cat back and checked for flow out of both pipes and found the driver side had significantly less than than the passenger's side. After checking further with a flashlight, I could see the broken piece. If it was one of the front cats, it would have been impossible to find with a flashlight, but the flow decrease would have helped find it anyway. It was still the stock h-pipe, so I switched to a Bassani X and was all set. Another option would have been to cut out the old cat and weld in a new one, but with the restriction of the stock h-pipe w/4 cats, the best thing to do is swap it out. Let me know what you find and I will check back on this post in a day or so.
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