Volkswagen Repair: loud resonnance noise from under side, tire store, catalytic converter

Question
1997 JETTA GLS 71K i HAVE HAD THE SHOCKS CHECKED, SUSPENSION, EVEN REPLACED THE MUFFLER PER THE DEALER (THAT WAS THE PROBLEM). NEW MUFFLER AND PIPES. sTILL HAVE THE NOISE. I TOOK IT BACK TO THE DEALER.  WE CAN NOT FIND ANYTHING? NOTHING LOOSE? cOULD IT BE THE CATALYTIC CONVERTER? HAVE YOU HEARD OF THIS PROBLEM? HOW DO I FIND OUT IF IT IS COVERED UNDER WARRANTY.

Answer
You told me everything except what kind of noise it is...mind you this is something I am a bit far away to answer.  Tires that are prematurely seperating can also make a low hum vibration type noise.  Check with a tire store to see if someone will ride with you.  If it is a rattling noise like marbles, it could very well be the cat.  Its more in your ballpark  Dealers can't find anything -they definitely don't want to fault their car.