Okay here is description my setup i need help with. I just got some megan racing headers and high performance catalytic converter and my civic i installed it myself except i have not done the catalytic converter yet because i requires cutting of original exhaust and dont really wana do it, after the headers going down there is another pipe with 2 holes leading into one and then its supposed to connect to catalytic but right now i have it connecting to original exhaust where it has some type of bracket where the old peice connected to and some type of round gasket, and my tail pipe isnt the greatest its just a checker auto kind got it when i first got the car.
So now to the issue of my setup...It sounds like **** when i get a little higher in acceleration it makes some weird high pitch sputter noise and i cant figure out if its the gasket connected to old exhaust maybe i didnt tighten it enough or if it my exhaust tip but it almost sound like a leak and when im in neutral and rev it i cant really hear the ****ty sounding noise. Ive checked for cracks and have not found any, i was gonna go today to a muffler place and have them make it sound good and install my catalytic but thought id ask around on what the problem might be and about how much will it cost to take it to muffler shop. also after installing the headers ive noticed a decrease in power, the stock setup ran better so any help would be nice

 

In my opinion exhaust work should be done professionally... Unless of course you know what you're doing. And you don't...