Just in the last couple of weeks, I've started to hear a sort of rattling
noise from the rear.  It becomes faster and more pronounced as I step on the
gas.  Upon closer inspection, it seems to be coming from the catalytic
converter which, I assume, is original equipment.  I have a 90 multivan with
about 148,000 miles and it was successfully smogged about a year ago so I
have another year to go before another.  My questions are:  Is this noise
typical for a failing cat?  Are they difficult to change out?  And if I just
live with it for now, will there be increasing noise or other consequences?
I did search the archives but these things weren't covered.

Thanks, Debra