Hissing from underneath

[Brian Paul Bressler <n9710420@scooter.cc.wwu.edu>]

I have a '85 4k quattro with a hiss coming from underneath the car.  I 
have only heard it when I'm standing next to it as it idles.  Could this 
be my cat going bad?  (i heard a uses car salesman use a similar 
excuse).  If it is clogged or bad or whatever,  who is a good supplier of 
high flow cats?
TIA
Brian Bressler
4ksq
Bellingham, WA