I watched your video twenty times and could not here it but then I watched the US Twintake one and heard it straight away.
Ill tell you what it sounds like, it sounds like the waste gate actuator is a bit dry on the linkage so when its opening to relieve the pressure its squealing against the pin and linkage.
As soon as it starts boosting that actuator starts moving in readiness to dump the boost out of the exhaust.
Here is the LO380 from Loba:
Can you see what I mean? I marked the points where the actuator moves in case you don't know.
Now the big engine cover and pre cat are gone the noise is going to be louder.
I could be wrong but nothing to lose take it all off, again, and where that support/heatshield sits have a mooch for the wastegate actuator shaft and give it lube with some WD40/GT85 refit and see what happens.
It will probably come back but at least you will know.
Its very dry inside that exhaust and around there so you never know but to me it sounds mechanical.