I had my frontpipe/downpipe and catalytic converter replaced last week. Ever since the work has been done I've had a hissing sound under acceleration. Don't hear it so much in 1st, 2nd and above you do say over 2500rpms. You can hear the engine but you also have the hissing sound, if you floor it to pass someone you can hear it. I believe it was an aftermarket downpipe and catalytic converter. Whats bothering me is the car never made a hissing sound before, all you could hear was the engine until the exhaust work was performed. The exhaust is quiet except for the darn hissing, going to try and go back tomorrow and have them look it over, just wanted some opinions if anyone knows what it could be, thank you.

 

A good way to check for exhaust leaks is to hang a piece of toilet paper from a length of wire. Hold that along the exhaust pipes and see if it blows the paper away from the pipe.
Does it still idle good? I wonder if they knocked a vacuum line off when working on it?

 

Probably a leak at one of the gaskets, hopefully they do right by you.

 

Went back this morning, they found a pinhole sized leak between the downpipe & cat, he welded it up and it sounds much better. Thank you for your help