Buzzing MBP?! Any Ideas?

Ok this has been bugging me for some time now... I have a 3-4 year old 15 mbp it has been solid, no problems whatsoever. For the past 2 weeks or so I have noticed a tiny buzzing sound coming from the screen hinge. The weird thing is that it always happens once I open up my lid but it goes away in about 10 seconds or so, another strange thing that happens once it begins to buzz I can make it stop by moving the screen to a "sweet spot" and the buzz goes away once that happens I move my screen anywhere and the buzz will not come back, completely silent. I can easily take it apart and poke around inside (I'm very comfortable in gutting computers anyway), but I just want some kind of idea of what to look for... could it be the fan? cpu? gpu? screen tubes? BTW the noise is comes from the area where the f8,f9,10 keys are at. Any ideas?