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That's surge for sure. Normal problem is that the BOV isn't opening in time and the pressurized air isn't being evacuated quickly enough. thus, that air looks for the best opening which is the turbine wheel and while going back through it causes it to slow down abruptly making the sound you hear.
If you have another BOV that you can swap in, try that and see if it fixes it. You can check to make sure that your BOV is working correctly by hooking up the vacuum port to a vacuum pump and pumping it to 13 inhg (inches of mercury). If the valve opens at that point then it is working properly. Are you running a recirc. setup?
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