Hi,
I had the same problem. As others suggested, it was the high voltage heater.
One service acknowledged the noise, but could not find the cause, but other service identified the part and completely replaced it. The exact part number is 1EA 963 581 F. I don't know if the whole heater needed to be replaced, but it worked.
Just some more info on my noise: this high pitched whine/buzz was only audible when driving in D/B or R. If you switched to N when driving it disappears, it is also gone when the car stops completely in autohold. It's back as soon as you touch the acc pedal. Speed or AC had almost no impact.
