Radio frequency interference (RFI) is a big source of all kinds of noises. Many RFI interferences are easily un-detected if you can't hear the actual "radio signal". Radio interference can easily take the form of hash that sound like white noise. Proper grounding and shielding practices are your only real defence against RFI. Digital noise usually takes a form of high-pitched whine (usually round 15 kHz or so), but at lowe levels can be mistaken for hiss. Digital keyboards, lighting, computers, effect processors and just about everything which uses fast clock pulses can cause this problem. …