I'm a complete 100% retard but check where the bov mates to the cold pipe, as that could've become loose, check your vacuum hose, other hoses, connections, and clamps. Oh and check all your boost connections.
Here it is, 8th post has the link to the tsb.
http://www.mazdas247.com/forum/showthread.php?t=123709505&highlight=mazda+6+tsb+technical+service+bulletin
Yes, I believe I know exactly what you're talking about. Sometimes do you also hear the same squeaky noises when you shut off the car? That happens to me too and I believe there's a service bulletin on the 6s with the same exhaust set-up. Undo your test pipe and use fine sand paper to clean both...
I love the 3rd gear just a little bit over 4th
Most fox-2004 mustangs with just intake/exhaust hate our 3rd gear...
3rd - Kill begins
4th+5th - Wraps it up
.....on the drag strip of course.....