Electrical noise on guitar pickups from computer. Due to radio interference, or inductive coupling?
There is probably some of both going on.  A vibrating guitar string causes only a small voltage to be produced in a pickup coil, so the output of these have to amplified quite a lot.  Due to the high gain, small amounts of noise that would otherwise go unnoticed become audible.

One way to tell the difference between magnetic and capacitive pickup of the noise is to shield the pickup electrically but not magnetically (or vice versa, but that's harder to do).  If the pickup is already in a metal case, ground that case to the ground of the amplifier the pickup is connected to.  That should significantly reduce capacitive pickup but not change inductive pickup.  If the noise is still there, then it's not from capacitive pickup at the coil.  That doesn't necessarily mean it's due inductive pickup by the coil since there are various other paths of noise into the system, but you've eliminated one possibility.