Same here, but it starts at 3000rpm no matter if you are on boost or vacuum. The sound also stops at 3000 if you are engine braking from the higher revs. I have to make an earthing kit first to make sure it's not interference as it sounds like it. It's also hard to say where it comes and I don't want to rev the car too much on our parking hall where sound echoes quite well. Timing belt kit, water pump and alternator has been changed recently, but I think it's not related to one of those even the sound started appearing lately.