I had turbo flutter on my Volvo and it was because the BOV wasnt opening enough or at all. The fluttering you hear is the pressure waves from a sudden throttle close bouncing off the impeller. Eventually can cause turbo damage depending on how severe/boost levels.

Not sure on your cash situation but a Megasquirt ECU will be your friend in the 420A world. Best mod, turbo or non, hands down. If you dont got the dough for that, at the very least get an SAFC. Those wont help as much as MS, but its better than nothing.

95 Eclipse GS: 10.5:1, Crower 2's and MSII
186 WHP - 150 WTQ
14.697 @ 93.85 MPH......Gone, but not forgotten
99 420A Avenger: Given to my sister
99 GS OZ: Sold
03 Evo 8: Family Car with balls