Why do I have this sound?

[Embalmer]

I just recently replaced my stock manifolds with FRPP shorty headers, and added an off road X-pipe (removed cats). This is bolting up to my 1 year old Pypes Mid-muffler system. The sound is the same as it was before I removed the cats and manifolds, just louder, which is good, but I have this reverb sound at 12 seconds remaining in the video that I can't figure out. My guess is the exhaust is vibrating back and forth at 2500 rpm. The system is only connected via the bolts at the headers and the hangers by the rear axle. There is no support for the x-pipe or bullet mufflers underneath the car whatsoever. Any thoughts appreciated.

http://s77.photobucket.com/albums/j56/faxisis/?action=view&current=090408001.flv

[Nino]

I could be the CMCV valves opening in the intake manifold or perhaps the cam phasers kicking in.

As a side note, the x-pipe should have hangers welded onto it to support it. If it doesn't, I'd have them installed at a muffler shop.

-Nino