Take a series of of bias frames and if they are relatively the same it is normal sensor bias. If there is a major difference it is EM interference.
How you get rid of bias etc.
Take flats and darks at the same exposure and temperature as your flats
Take darks at the same temperature as your lights.
Take bias frames as well if you need to remove noise from your lights with darks at a different temperatures.
Here is a single exposure at 1min at f/5 at 300mm with 1600 ISO corrected for darks and flats and stretched. 1.3MB
http://d1355990.i49.quadrahosting.co...T_IMG_0056.jpg
Here is a good start
http://www.saratogaskies.com/article...ook/index.html
Bert