Do you hear the surge valve opening once you let go of the pedal?
Do you have modded the Boost controller?
Do you have a boost gauge? If you do, what are your readings?
This may sound crazy but, maybe it is not under acceleration, maybe it happens when the Throttle Body closes and the boost inside the line pushes it out. the turbo generates 14 PSI above absolute presure("0" PSI) but in reality you are making 29 PSI absolute (this is with the addition of the 14.6PSI that is ambient air at sea level). this is a lot of boost compared to a N/A car with -5 Absolute PSI intake.
The reason for the factory Surge valve is, to open once you let go of the throttle, although poorly, it lets the excess pressure get out. If it's working in a less than optimal manner, you can have excess pressure left inside the intake hose and push it out little by little.
This is just another angle to your case. It doesn't mean is the problem.
Ian
__________________
|