Detonation?

[opponheimer]

So about a month ago I let a friend of mine drive my s14 (s13 sr20det w/ gt2871r w/ 740cc w/ bpu) while I sat in the passenger's seat. The car is on 20lbs of boost and runs 93 Octane all the time with enthalpy rom tune. AFR at WOT is 11.3-11.5:1 always. UP around 4500rpm - 5500rpm in 4rth gear in WOT we could hear a quiet little noise that resembled bb's hitting a metal shield or something, it sounded like detonation. After 5500rpm it would stop.

Timing is 15 deg.

Anyways, I lowered the boost dramatically to 15lbs and I still hear it. Could it in fact be something else?

My Knock sensor is in good working order and ecu throws no codes, so if it was knock, wouldn't the car have retarded timing if it really was knock that was loud enough to be heard by human ear? The car runs flawlessly regardless.

I am almost wanting to make want of those home testers you bolt to your block with headphones and tubing. Anyone have any advice on where to look, or absolutely know what this is, than let me know. Thanks!