Ghost in the machine?

Try unplugging everything and then see if you are getting any noise through the speakers. I have a neighbor with a ham radio, and every so often I will pick up noise from that. If you are using unshielded cables, they may be acting as an antenna.

you have a router, don't you?

Yes. I have a router. 

the noise may be from the router "hunting"

you could turn it off and test

or perhaps move it as far away as you can