i came home yereday after driving 100 miles with my car... on a trip and when i get home i could hear a cracking sound under the car kinda like a rolling boil .....i know my cat is bad i need to replace it it was ratting loud a week ago however it is a very sigth tapping rattle noise now so was it my cat making the noise? thius was with my car off...

Probably expansion/contraction of the exhaust system, normal.

Probably expansion/contraction of the exhaust system, normal.
its not normail for a cat to rattle or make noise period in my opiton

its not normail for a cat to rattle or make noise period in my opiton

He was saying the snapping noise you hear is normal, it is the exhaust sytem cooling and contracting.

He wasn't commenting on your cat.

Don't know if this is the same deal, but a while back, I had a metallic rattle from that area that was driving me nuts! Mechanic told me it was the cat, but I didn't have the $$ to replace it at the time.
On my third time under the truck myself, I found two bolts on one of the brackets that support the exhaust (forward of the cat, if I recall right), that were loose & letting the assembly rattle. Tightened them up and the rattle magically went away.

Just an idea...

-Mike