A month ago, I put on a new cat. sounds great at idle and throughout the rpm range, but when I rev it up to around 4k, and let off the accel, there is this terrible rattle. There is also a rattle when the engine slightly bogs, like when I start, or when I shift into 2nd at a low rpm. I hate the rattle, and it makes me hate my car. I just changed the tranny mount, and it did nothing, its still there. Please help, someone.

 

Is it hitting the heat shield?

 

no, I think its the y pipe, it sounds like something is inside it, but I dont know what it could be, i know its not hitting the heat sheilds though.

 

there could be pieces of the old cat in the exhaust,but i dont think it would be in teh y-pipe b/c thats in front of the cat. try gettin underneath the car (when its cool) and pulling on the exhaust and such to make it rattle or see if anything is too loose.

 

crawl under the car with a rubber mallet and start banging on stuff. You'll find the rattle pretty quick.