I'd love to cross post this to death - but the true Guru's are more likely to see it here. The car is an automatic which makes avoiding these conditions difficult.

Mods that matter: SZ55, EWG w/ 1.3bar springs, TMIC, JDM STi BPV.

Conditions: While in top gear (4th with converter locked) if I lightly tip into the throttle just enough to accelerate slowly while building boost (~3000rpm and 10-12psi) I get the strangest compressor surge - very slow flutters - as soon as I slightly back off the throttle - but not completely. i.e., the throttle is still open enough to allow me to continue at the current rate of speed but not decelerate at all. Flutter begins and boost drops - I hear NO recirculation.

In short: I hit 12psi in top gear (~3000rpm) accelerating slowly, back off throttle slowly to maintain current road speed - no recirculation - half a dozen slow "chirps". Otherwise BPV seems to work normally.

BUT - the car hasn't been tuned for the SZ55 yet (left with the tuner today) - so I haven't had a chance to properly diagnose this issue.

Dude, next time try the -SEARCH- feature. In short, it appears I'm reaching the compressor stall threshold when I'm "lugging" the engine at 3000rpm and 10-12 psi. The way to fix it - avoid doing the previously stated or completely dump the throttle - don't leave it cracked. It turns out the larger turbos are guilty of this more often than you think. Here's a link to the thread that answered my own question. LINK (http://forums.nasioc.com/forums/showthread.php?p=11542242)