its definately something loose that is causing the rattle, for me, i get rattle from the cat sometimes, i think theres rocks that are caught up there. havent had the time to investigate.
its unbelievable, that there are so many useless mechanics out there. before when i gave cat bolts to the mechanic to put on, he said its the wrong one and doesnt fit, i later discovered he was trying to put the spring between the cat and the exhaust pipe when its ment to go behind it. I just put it on myself at home, wasted 50bucks for his bs.