Hi -
Smog checkers don't like Cat Conv rattles, what happened to mine was 
that the Cat grid has started to melt from use and pieces had broken 
off and stater to rattle.  The rattle sounds tell the smog checkers 
that the converter isn't working completely.  Pieces of the CAT 
breaking off can damage the muffler also.
Take the unit out and get rid of the rattling pieces.  
BTW, I've heard that the cat material has a decent resale value to 
metal scrap yards due some exotic metal in there.  On one of my 
busses, in a fit of desperation, I completely removed the contents
of the Cat Conv, (trying to solve anoter problem) and it still 
passed smog in AZ, retained the housing so everything still fit. Just 
nothing inside!
Liquid Wrench is still the best bolt unlocking compound IMHO.
If the rusted bolts are truly bolts and not studs, get good wrenches 
and twist them until they break, or cut off with saw.  Not worth 
saving them.  Work a LOT harder to save studs and inserts.
Jeff - '80 Westi

david wrote -
> 1) I've got a rattle in the Cat. converter.  Do I necessarily need to 
> replace the converter?  I know it won't pass emissions inspections 
> without it, but can I damage the engine by not fixing this?

> Sort of 3rd questions - can those rusty bolts be dealt with?