Sorry to resurrect an old thread but I had this exact issue yesterday and wondered if anyone had any thoughts or solutions.
Basically, after a couple of short WOT bursts the BOV started to sound odd, giving off noticeably quieter and shorter releases of pressure. The car continued to drive fine, pulling and boosting correctly as far as I can tell, and after half a mile or so the BOV returned to sounding normal for the rest of the drive.
I’ve checked for vacuum leaks and found nothing obvious, though haven’t had chance for a thorough look. I did think about just ignoring it as it seems to have resolved itself, but it’s now bugging me a bit! If anyone has any ideas as to what happened then I’d be most grateful. Car has BPU and stock turbos.