I've been chasing down a nasty rattle, and I'm pretty sure i found it in the two upstream cats. I have a stock midpipe, 70,000 miles. When I hit those two cats, they make a funny rattle sound, much like the sound I've been hearing. I'm thinking about getting a new midpipe w/ cats. H pipe or X Pipe ? I need one that won't contribute to any drone, if possible....