GMac13

Glad to help It drove me nuts also, cause I bought a used superb with 80k km on the clock and it started doing this. Dealership in Lithuania is amazing. Mechanic sat in the car with me and immediately said that this is a strange noise and it can't be there. Took the car to the lift, and with a set of stethoscopes after an hour found that this solenoid housing was touching the firewall. When solenoid started to work with a low frequency like 50 Hz, it resonated to the chassis and only inside the car You could hear it.

I had same noise. Dealership found the issue. Turbo solenoid was loose. After acceleration solenoid starts to work and resonates to the chassis. Simple fix, tight that solenoid bolts. If that helped, You own me a beer