If you don't act on this then you're stuck with it later. I don't what hum maybe other than the cpu fan could be it. In either case, the warranty is there to help resolve issues. I suggest you also check the HP forum on their support website and see if anyone else is experiencing a similar issue. Also be darn sure no "audio file" is somehow enacted to emit some sound, though I thought it.
I got a new HP Pavilion dm4-1201us three days ago and, thus far, it hasn't stopped humming when it's turned on. It might be the battery, though the strange thing is that the computer is not hot at all. I also tried to close all my programs (to see if something I opened was just taking up too much space) but the humming only dimmed, and was still pretty loud.
I could still get an exchange, but I'd rather not since I already bought Office and installed many of my other programs.