that's the lil headphone gnome that lives inside your headphone drivers hummin along to your tunes! you must have good taste in music
lol. see if the humming occurs when you are using the headphones by emselves or if the gnome just doesn't like to play nice w/ your amp or dac. check your cable connections & source. the headphone gnome likes to have everything very tidy or else he gets cranky.
I had a problem similar to that. I did everything i could think of, changed headphones, amps, dacs, computers, chenged ports, used different plug sockets etc. It drove me nuts. Strangely imo, when I changed the cables between the dac and headphone amp, the problem disappeared.