The sound is coming from the heat sink and fan. When the graphics chip becomes hot the interface (or mount) between the heat-sink and fan becomes warped. The heatsink and fan can transfer the heat load very quickly. So when you stop playing your game, or even pause, the temperature drops rapidly and the fan mout is no longer under stress. Now if you want to prove this to yourself, open up the case and you'll need the help of a friend and a can of compressed air. Be very careful with the compressed air. If you get too close it can damage the fan. Would you want to do is win you get to the point where this thing is buzzing blow just a little bit of cold air into the fan area and the buzzing should stop. Please be careful not to blow the fan very hard and damage
it. Just add a little bit more cool air for a couple of seconds and the thing should stop buzzing almost immediately.
please let me know if my answer is absolutely wrong, absolutely right or maybe close..