Not to freak you out or anything but that's what my stock turbo sounded like before the shaft snapped in half. I thought it was like a belt slipping or something.
i would assume the noise would have been from the shaft allowing the wheels to contact the housings? as the turbo and engine rpm changed did the frequency vary? mine doesnt sound rpm/frequency dependant its kind of the same tone any time it does it. i did a hard pull 6th on the highway on my way to work this morning and got 18.8psi and no noise. later i did one in 3rd and then it did it. if the rate of accel is causing enough of a pressure drop through the throttle body im thinking the bpv may be opening slightly? has anyone experienced this happening before?