Exhaust "clanging" noise

[1983Pinin]

Hello,

My 1983 Pininfarina Spider has an exhaust system clanging noise that you can hear if i bounce the tailpipe by pushing on it with my foot. I've gone underneath and it doesn't appear to be a pipe hitting anything -- just metallic banging noises that stop once the pipe stops bouncing. It sounds like it's coming from inside the rear muffler-- the one with the pipe that goes over the rear axle.
Before I go out and buy another muffler and install it, can anyone confirm if internal things in the muffler that can shake loose? Do these things fail in that way? The rear muffler looks really good on external inspection, not rusting out or anything, and I really see can't any pipe banging anything underneath ( although it does sound like that ). All connections in the exhaust system are welded and tight-- it still has the catalytic converter. All rubber hangers and catalytic converter springs are in place and good. The exhaust system seems tight-- when I try to bounce it with my foot to make the noise, it's hard to push--not much wobbling of components.

I am just asking is it possible that something inside the muffler is rattling?
Has anyone had experience with this?

Oh-important note-- it makes this noise when idling--usually right when I start it.

Thanks
--Peter

UPDATE!!! I finally removed the rear section of the exhaust system, and when I got it off I found the clanging noise was in fact coming from the resonator. So nothing rubbing or hitting the exhaust system. I didn't bother cutting it open to see the cause, but as mentioned in one of the replies here, sometimes the internal baffles can come loose. Funny thing is the outside of the pipe and resonator look like they are in reasonable shape. No exterior rust on the exhaust system, so something just let loose internally.
--Peter (Sept 2018 )

[joelittel]

Mine made a similar noise, which ended up being the parking brake cable bouncing off part of the exhaust piping.

I know that you probably already have, but I'd say before getting a new muffler I'd check every possible culprit.

[Odoyle]

The exhaust on my 83 does the exact same thing, in an attempt to address the some of the plausible causes, I have replaced the catalytic converter springs, muffler hangers, and bracket which secures the manifold to the bottom of the trans housing, and it has helped to reduce the unpleasant clanging on startup or at low rpms, but after a more thorough inspection, it appears that their is something that is rattling in the center muffler (after cat) and rear muffler as well. With that said, both mufflers should probably be replaced. On a side note, how much exhaust back pressure should my car have (not sure how that is measured) because it seems like their is a fair amount on my car, and is there such thing as too much?

[So Cal Mark]

many mufflers have baffles in them. If you still have a catalytic converter the catalyst bed may have broken loose inside the shell