It could be a few things:
mains hum, and spill from other electric equipment sharing the same wiring - use a signal conditioner (spike protection etc) on your power cables. Quite common to spill onto mains/grid power are things like central heating / air con systems, and people turning things on or off. Mains hum itself is low and the audible frequency is low ~ 50Hz in the UK, but as mentioned other signals can spill onto the ground circuit and pass to other equipment.
crosstalk (route your audio cables so they are no where near power cables, digital audio or midi cables, use better shielded cables)
crosstalk within the circuitry of your recording gear
radio interference (a form of crosstalk) could be picked up by your cables, or your guitar pick ups.