Hello,
Just additional info. The Bosch waste gate valve in Volvo turbos is the compressor-bypass type (CBV). It vents excessive pressure back to the intake hose during idle and foot-off throttle changing gear time. It doesn't vent off to outside atmosphere as the blow-off type (BOV).
Please refer to this Volvo turbo page:
http://people.physics.anu.edu.au/~amh110/blow_off_valve.htm
The CBV could also in need of adjustment.
Please refer to this page:
http://forums.swedespeed.com/showthread.php?104485-Wastegate-Valve-Adjusting
From your description it could be the culprit. If you have a spare one replace it temporarily before getting a new part. Also please check that your air filter is not clogged. Otherwise it can't vent off the pressure.
Hope this helps. Good luck!
Amarin.
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