I have a testpipe on my car right now and the CEL keeps going on .. so I'm planning to swap it back the old cat and run my new exhaust with a cat. (Greddy Ti-C) would putting the cat on not only make it quieter but create more drone?

 

yes it will increase the drone and make your exhaust quieter from the outside.

don't take off the test pipe. just get a spacer/anti-fouler to hold the O2 sensor further away from the exhaust flow and you'll be good to go.

 

okay I will fix the bung (or try to) and run a testpipe with the exhaust

next question ... I realize that a testpipe with oem exhaust is suppose to sound like a tin can .. but mine seems a bit loud (loud as in the tin can sound is quite excessive), almost as if air was seeping out somewhere. my friend that helped me install it said he screwed all the bolts on very tight and that there shouldn't be any air seeping out ... any ideas?

 

maybe get new gaskets and see if that helps?