Totally agree with harmsangha.. it is NOT the fan, from my experience with turning on and off the automatic graphics switching i think its the CPU graphics component since the noise stops when automatic graphics switching is turned off (thereby de-activating the Intel 4000 integrated graphics - the culprit).
This is a very premium computer and it should be perfect, the genius bar is rarely helpful in these matters but you have to keep trying, go to another Apple store or go through Apple Care and complain until they help you. And again, make recordings video/audio whatever you can, it is always helpful to have evidence of your problems.
What's weird for me is I dont know if I was unlucky to get 2 computers back to back with the same problem, or is this more widespread and some people are just sticking with it/ thinking its normal? I've been lucky to speak to a helpful apple rep at Apple Care regarding this issue, but I've experienced awful customer service from senior Apple Care reps in the recent past and sent in my complaints.. why would I want to return my brand new computer for another one unless there is definitely a problem?? Sometimes they make it seem like we are scheming which is utterly ridiculous - I dont know about you all but I think its a hassle having to return a comp wipe it clean get a new one and re-install everything through time machine... hours and hours of wasted time.