2013 X1 whistling noise

My 2013 X1 has a whistling/whining/whirring noise that seems to be coming from in front of the driver's seat but in the engine. The dealer repair shop thought it was an air leak in rear view mirrors so they added a seal. Noise still there. I've returned several times and the last time they kept the car almost three weeks: replaced all the vacuum hoses to the turbo and even the turbo itself. Noise still there. The noise sounds like a turbo whir (like some older SAABs) and is clearly related to acceleration since it stops when you take the foot off the pedal. One mechanic on a test drive described it as "rotational" and that makes sense. I'm talking to the dealer about a trade or a buy-back. They want one more attempt but have no reasonable hypothesis as to what is causing the problem. This has been going on for over six months. Has anyone dealt with this? Any suggestions?