the fluttering is compressor surge from your turbo. as you shut the throttle body, the boosted air has no where to go, this is where the BOV vents it back into the intake. if you have a BOV and haven't done anything to it, then it's the bov just venting back through your intake. if you don't have one, then it's compressor surge, and thats not a good thing, as can damage your turbo.
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