I've had my 13' ST since new, coming up on 100K miles, and recently I've started hearing a high pitched wine/squeal which seems to be coming from the passenger side of the engine compartment. What baffles me about it is that it does not change pitch regardless of engine speed or vehicle speed. However, it only seems to come up around and above 2K rpms. It only makes this noise about 2/3 of the time. It will make the noise if I rev the car up above 2K rpms and hold it. I've reasoned that it likely isn't mechanical, like a bearing, pulley etc. Nor is it a bad transistor, ground, alternator etc. since those would change pitch with engine speed (I turned off the radio and HVAC system which made no difference to the sound). Maybe its a vaccum leak, PCV or ruptured BOV diaphram? I haven't noticed any other issues like low boost/power, so I'm frankly at a loss for what this sound is and its driving me crazy. I would like to at least have some idea on what this is prior to taking it into a shop, so anyone's help or suggestions would be appreciated.