to joep: think I've ot that problem
What you described (hit on cat casing) and chunk in muffler) is what I thinlk I've got.
I have a '77 450 SEL (Euro) ith fitted cats. When I got car, I replaced left one with universal. That was about a year ago. Ran fine for many months.
Then, in recent couple of months, I have developed problem. Car will rev fine and then develop a restriction-like thing on left side. I looked under the new cat, and saw a dent or crease. Can't remember left side hitting anything, but there it is. There's also something jiggleing around inside that cat. Muffler sounds a bit rythmic but not overly noisy in rear. It's fairly new. When I stop car, and thenr estart, it goes away for a bit. Maybe a couple of minutes at most. Worse under load, to point that I lose almost all power and it's an embarrassment on road.
Everything else has been ruled out.
Could a chunk have been broken off and be lodging in rear muffler or resonator? Is there any way for me to try and remove or break-up the obstruction?