Awful Exhaust Noises

Currently I have an SR Motorsports catback and an OBX header on my MP5. Since I put the header on, there hasn't been a good seal between the header and midpipe until just recently and now that it is sealed it sounds pretty awful under heavy throttle. When I give it a lot of gas there is a really obnoxious rattling/buzzing sound. Like I said, it was just given a proper seal and also the cat was just replaced because a piece had broken off inside. Is there anybody that has had these same two exhaust pieces or experience with this noise? Any advice on how to fix it would be much appreciated.

Have you looked at the heat shield? With the work that's been done on the exhaust recently it's possible the heat shield got bent and is hitting the exhaust on heavy throttle.

I have a TWM short shifter which requires the front of the heat shield be lowered about an inch to where the bottom of the shifter and the top of the exhaust are practically touching. As a result, they often clang together when it's just idling so I know that sound and I don't think that's it.

are you using an OEM donut gasket? when i had my 2 different headers i noticed the ones they came with were pretty crappy.

The header actually didn't have a gasket when I got it and I used it without one for about 2 months. The shop I just took it to to replace the cat put a donut gasket on there, but I doubt it's OEM. I'm not sure what brand it is.