Ive had a couple of bikes do this to me. Its not gona bother anything while it does it. If you notice it will only do when you A) leave it parked out in the sun for a little bit or B) when you have been ridding realy hard to a long period of time, normaly over three or more hours. Ive been living with this sound for three years on multi bike and have found that the only way to stop it from doin it anymore is to replace the gas cap, the hole thing including the rubber seal. Just so you know, the cap I replace with a factory replace ment ran me 100 bucks for the cap, 10 for the seal and I had to change over the key and and lock before I could use it.
My advice, live with it, no harm will come of it.
Josh