OK...here are the sounds and what it is:
ONE big WHOOSH or PIISHTTT when letting off the pedel under load = Is the excess pressurized air....and totally normal
Anything like WEEEEEEEEE (high pitched) = normal. Let that turbo sing!!!
MULTIPLE and VERY fast WHOOSHes or PIISHTTT's = Compressor Surge or Stall. This is NOT good. Fix is either Wicked Wheel and ATS housing or an aftermarket turbo.
Anything like a CLUNK CLUNK CLUNK or metalic sounding = BAD!!! Go get a new turbo
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To kill a few myths/wrong thoughts:
Wastegates are on the exhaust side of the turbo...they re-route the exhaust gasses around the exhaust wheel so that it isn't spinning as fast, therefore, less boost. Deleting the WG CAN cause over boosting and toasting your turbo if you are not careful. :SHIFAN: (I do not have a WG anymore.)
Blow off Valves or BOVs are on the intake side of the turbo...after the compressor and before the THROTTLE plate. (Diesel's do not have a throttle plate.
) They operate on vacuum on the opposite side of the throttle plate. When the plate closes, it creates a vacuum between the cylinder and the plate, this vacuum is read by the BOV and the BOV opens, releasing the excess boost pressure on the pressurized side of the plate. This stops the turbo compressor from having to try and slowdown to vent the pressure, keeping shaft speed up and allowing boost to be made immediately when the throttle plate is opened again.
These are not technical definitions, these are personnal explainations of how I understand the workings of turboed engines.
