It's not static if it happens with all sources (about 80% sure on that one) and not cabling (I agree w/ Porziob) then I'd guess RFI -- probably not a faulty ballast however (although could be, but that's not the kind of RFI they produce.)
I'd guess some kind of device with an arcing relay, like a fridge or airconditioner compressor, or an electonic ignition device like found in a gas water heater, furnace, or stove, IMO.
I'd guess some kind of device with an arcing relay, like a fridge or airconditioner compressor, or an electonic ignition device like found in a gas water heater, furnace, or stove, IMO.