Possible detonation?
 

Today I did a few WOT pulls to 7k rpms, on which I suddenly heard what sounded like a bunch of rocks hitting the drivers side firewall/under-body of the car. It was extremely loud and re-produceable. I thought I left a bunch of bolts in the engine bay. It seems to happen right before/as I am lifting off the throttle to shift.

If I could pin-point what the exact sound is like after doing it a few times, I would say it sounds like backfiring out of an exhaust leak in a thin-walled header/test-pipe. It's an extremely loud tap, non-consistant, as if I'm getting rocks kicked up at me.

I know this engine is noisy, but actually overpowers my already loud exhaust. It got worse as the night progressed when I would let off the gas around 5-6k rpms.

My ultragauge reports no pending codes and the car runs fine. I run BP 93 octane and I was thinking it was bad gas, but the deceleration part is what gets me. Could I be drinking oil on decel and igniting --since it's a rev up?

I wonder if a misfire code would show up if I was detonating that hard at a high RPM/load? I am not tuned yet.

2005 Track (revup)
Stock airbox
Mrev2/Z1 spacer
Megan V2 test pipes/OBX y-pipe
Nismo Exhaust.
Iridium plugs (1-step colder iirc)

Thanks for any input in advance!