To my ears it actually sounds like compressor surge, the thing that a BOV is supposed to eliminate. It's the coolest sound a turbo engine can make, in my opinion, but it's actually damaging the turbo. I recognize the sound because for a while, on a previous turbo car, I drove around for a while without a BOV when I knew I was going to upgrade the turbo so didn't care about the OEM one and wanted to enjoy the sound of the compressor surge. It sounds much cooler than any properly adjusted BOV.
I'm pretty sure that a properly adjusted BOV will make a single sound, be it a whoosh or a whistle. Any CHO CHOO Choo choo sound is compressor surge.
I wonder if the BOV in the video is just the best sounding unit I've ever heard, or adjusted too tight and it's not opening fast enough to vent all of the built up boost.
The first time I ever heard compressor surge was the old Audi IMSA GTO cars. I would hang out at the braking zones just to hear them come in, compressors chirping away. Always made me smile. I asked one of the guys in the Audi pits what that sound was and he said compressor surge from the turbo. I had no idea what he was referring to back then and didn't want to pester him with more questions so I just nodded.
J