You might have a bad exhaust gasket and its leaking.. My care was fine With the Stock Exhaust changed to a cat back and the same noise u guys have i have.. needless to say i threw the stock gasket away since it was falling apart... I have a new gasket coming to morrow and it should slove the ratting sound cus its making me mad... its really bad on morining starts.. try looking at the gaskets.. im sure if its from the exhaust its a bad gasket or u need to replace the catalyic conveter it might be damaged and making a rattling sound
