Everytime I open up my macbook, the fan starts to kick in reaching up to roughly 2700 rpm according to istat? Initially, the fan will kick in but the rpm's will eventually hover back to the normal 2000rpm range. I should also note that this only happens when i'm not connected to a power source. That is, when I open up my macbook without a power source, the fan will kick in as apposed to opening it up while connected to a power source, the fan will not kick in. I was just wondering why that is. Thanks!