Strange buzzing when reving 2.0 crlb.

[Frostbite]

Hi, not been on in a while

The derv was doing a regen after a short drive home from work so didn't turn it off, after the revs dropped I blipped the throttle and could hear a buzzing sound (think a card flapping in the spokes of a bike)
Anyway with the bonnet open it's was very pronounced.

I haven't had anything done to the car so all stock, I did block the silencer chamber in the airbox with block of foam but I can't see that making a difference.

Any ideas?

Is the wastegate open for the soft limiter

I was parked next to a fence so maybe it was amplified.