insight into diagnose a problem on a reanult laguna 1.6?

i had the back box done a few weeks ago. since i have had this done i have heard this nosie wich has got loauder and u cna feel it vibarating under your feet once u reach about 30-40 mph. i have been told it could be my wheel bearing or the catlyst on the exhauste. is there any insight to this problem before i put it into the garage, and get told it 1 problem then get told it another problem

tracy
October 2007