My guitar has mains hum!!??

Step#1 - turn off any fluorescent lights

Step#2 - move away from your computer

Step#3 - slowly spin through 360 degrees and note null points

Step#4 - mark floor with masking tape line showing quiet spot

What causes the humming/buzzing?

Every changing magnetic field in your studio. The mains wiring in your house generates a magnetic field. That's probably where the hum comes from. If you have fluorescent lights there will be a ballast reactor generating both audible and magnetic buzzing. Various power supplies in your studio also generate magnetic interference that your guitar is more than happy to let you know about.

Spinning around in one spot will let you use your pickups as a directional field finder. There will usually be a sweet spot in every room where the racket is a minimum.

This is the bane of single pole pickups. They sound great and they're prone to field pickup.

What does this picture have to do with the text?