You do realise that air cannot pass through the compressor side of the turbo onto the turbine side right?

The wastegate has nothing to do with the intake or charge air. The wastegate controls the amount of exhaust gas that flows through the turbine housing, which in turn determines the speed of the turbo and the amount of boost created.

DL's explanation is accurate. The flutter is more apparent on smaller or highly efficient turbo setups because they spool quickly without a large amount of load.

The T28 is a small turbo so it only requires a small displacment engine to spool it quickly. I get a lot of flutter on my falcon since the GT3582R and 4L motor is a similar turbo-motor ratio