Car is working great, makes good boost, but when I let off the gas, the turbo makes this sort of high pressure release sound that reverberates for a few few cycles (like for a couple seconds is all)....then all back to normal til I hit the turbo again. Like a waste gate maybe?

 

Couple of engine codes I can addd to this. P0171 and P0507; System too lean bank 1 and Idle control system RPM high, respectively.

 

Surely someone has seen this before, but maybe I am not explaining it well. So here's another try:
When accelerating, turbo boost comes in normally and actually peaks a little high, over 15 PSI, but settles into 13-14 PSI. Everything feels and sounds right, until I let off the gas. Once I let off, there is a loud (even with windows up) high pressure release. Interestingly I just noticed a week or two ago that it sort of reverberates, like a harmonic in a power amplifier. I hear maybe 4-6 reverberations of the high pressure release, then it is back to normal. Also throwing these two codes: P0171 and P0507

 

Sorry I'm not an expert, but I've had the "too lean" code which turned out to be several vacuum leaks. Which can force an idle-up to compensate.
Personally, I'd ignore the noise and focus on fixing the sources of the error codes. That may well fix the noise too.

 

Ordered a diverter valve and the fluttering is gone. Pretty simple install, just tight confines. Now to see if the engine codes go away and stay away.