@johnzic,
There are a variety of sources. One such source typically is anything electrical that can create a electrical field as A/c hum (60 Hz) often creates line noise.
Another source for noise is atmospherics ..such as lightning and even sunspots when we are in active sunspot phase.
Another are rf generators (radio frequency) such as transmitters; fluorescent lighting, and lastly, certain electrical generators in the local environment that create sparks.