I know that sound.
It started after my turbo was done, but only sometimes and under full boost (yep around 4k).
If I take my foot off the accelerator, the noise disappears. Then it says gone when I put my foot back down.
My mates RS is starting to do it (so he says, I haven't heard it).
It is not possible to replicate as it happens to me us at random times.
No Danny, I do not know what it is!
Thought it might be an oil supply prob to the turbo, nothing to do with belts