One good way to wipe out a converter is to have a bad miss and run the engine for a long time. The converter loads up with unburned fuel and gets so red hot that the ceramic cracks, making that rattling sound. Eventually the broken peices rattle around and get stuck sideways in the outlet of the cat, not allowing exhaust gas to escape.
Sort of like what I did to my 66 bug when I backed it into a drainage ditch and smashed the exhaust pipes so bad that no exhaust could get out. It ruined the motor.
A real mechanic can measure the back pressure, I think the limit is about 5 PSI. I have a home made tool the I made out of an old oxygen sensor, it screws into the oxygen sensor hole and has a vacuum hose fitting welded on it. I screw it in and hook a cheap old fuel pump gauge to see if there is any back pressure in there.
I agree with the other guy who said its more that likely a vacuum leak problem than a converter problem, but you will figure it out.