Muffler Bracket/Clamp Problem?
 

On My 2000 Boxster S, when I shut off the engine there is sometimes a slight rattle from the back of the car. Might this indicate a loose or broken muffler bracket/clamp? No noises occur when the car is running. Thanks!

If it's the stock exhaust you could get under the back and try rocking the exhaust from side to side..
I serious doubt it's a loose bracket. There are two bolts securing on top, and and a U clamps on each bracket on the side. Then there is a coupling clamp on each end of the muffler.
There would have to be a lot of loose bolts..

Yes, the stock muffler. I'll give that a try, thanks!

Have similar sound with my 2001 base, but more substantial than a rattle sound, maybe and engine mount on the way out?

Check the heat shields over the rear cats. Fastners rust away and the shield just lays on the cat. I reache dup and both of mine were just sitting there with no fastners intact. now sitting in corner of garage :)