Is it possible that the Cat's substrate has actually broken loose from the can? I bring this up because this happened recently on my dad's F-150 and it had exactly the same symptoms. We narrowed it down to the cat by listening carefully, but nothing we did would get it to stop. Was still under warranty, so had the dealer fix it.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]