The hum you hear in your amplifier is usually caused by stray magnetic fields generated from the output and power transformers in your amplifier. It is commonly called 60 cycle hum and is one of the major problems that occur with single coil pickups. There are various single coil sized humbucking pickups on the market that can be used to reduce unwanted hum in your guitar With standard single coil pickups as you change direction to your amp, you change the phase angle in relationship to your instrument. If you are near computer monitors, radiation and internal electronics can transmit unwanted signals into your instrument.