i have a mk2 ba xr6t auto, some mods aren't really known as it was modded before i bought it, took it to elite automotive and they said it was a little lean but not unsafe, non the less, today it started to flutter under boost at lower boost it wont flutter, but when boost gets higher it flutters, it has gone down from 15psi to 12psi since the flutter started.

any ideas on what could be the problem?

What's this "flutter" your talking about? Could be a leaking blow off valve
a small boost leak, waste gate could be partially open.....

The easiest way I can explain the sound is like blowing through a straw an waving a finger past the air coming out
Or to have an air line and blowing something out to clean.

It's hard to describe the sound

The car used to reach 15psi but has only reached 10 or 12 psi since the flutter started

Sounds like a f**k bypass valve? Have heard that they do wear a fair amount in a short time.