I really enjoy listening to turbos spool up, blow off, and mis-time the engine. But one sound I've always wondered is how they get that flutter when they let off the gas, is it a good or bad sound? As in bad for the car or good. I don't want to set my car up to make that sound if it's a really bad sound to be happening :/.
Those flutter caused by compressor surge, more likely because the car do not have BOV, most older turbo cars running low boost without BOV from factory tend to make flutter sound when lifting off, gear change, or slowing down ( partial throttle ). You should not have flutter noise when BOV is fitted, BOV / diverter valve is used to reduce/eliminate compressor surge.
Another cause is mismatched turbo and engine, flutter or compressor surge could happen even on full throttle ( open ), usually because the turbo spools up much quicker than the airflow to the engine - turbo operating in surge region even on WOT. BOV won't help in this situation. This can happen when trying to improve spool up time, using large compressor on small turbine in order to improve response on lower RPM. There other ways to eliminate the surge caused by this kind of mismatch by tuning the engine to fit in with the chosen turbo.
Compressor surge won't cause extreme outcome when it happens mostly on lift or closed throttle, usually increasing wear on the turbo bearing and not so good throttle response. When it happens even on open throttle ( like the example cause by mismatched turbo with engine ), stuffing lots of air quicker than / before the engine can take it, at higher RPM, then it can be more serious, damaging turbo compressor wheel or much quicker wear on turbo bearing.
Boost level also a factor, the higher the boost, the greater the impact of the compressor surge.
