Guys, I have a 4S and I was at the library and I thought I was hearing the air-conditioning system, but realize this low whistling was emitting from my 4S. The best description is it sounded like low frequency radio sound waves. Has this happened to anyone? Anything worth investigating further? I don't have Apple Care, but still have until early November to take care of this if it needs taking care of under my warranty contract. Thanks.