The burble is always going to be there until you either drop more fuel pressure out of it, or take some fuel away from the stock injectors. If you listen to all the boost it videos of them creeping around they have the same exact burble as your talking about, same with the Vipec videos I have watch, BD or Silber they all have it.
Someday there will be a fix for it, I have tried to tune it out of mine but have not been able to. Talked to Silber about it and he claims he has been working on it
My sled, has no burble wut so ever. If it does, it's on the first day of a trip. I jus tune it out and I am done tuning for wkend. I can vouch for my BD riding partners, they haven't touched box all yr. there sleds run awesome, no burbles.
I am pretty sure everybody kit doesn't have a burble.