A friend of mine, an accomplished digital shooter in his own right, actually designed chips for a living about fifteen years ago. Some of you gamer-types probably remember 3DFX. Chances are if you had one of their cards, there was a chip or two he designed on it.
Anyways, he's the one who told me about electronic interference affecting the quality of a digital image. We did some very unscientific testing with his D30 (yes, D30) one night (years ago!) and found an appreciable amount of noise close to a speaker rather than far away. Yeah, we're that geeky. "OK shoot here, then move ten meters that way, and shoot there..."
Not enough to go "well the heck with this! Send it back to Canon!" but enough to prove the point.
Only way to "deal" with it was to thoroughly calibrate the sensor somehow, which DSLRs don't (and now don't need to) do.
At the time I ran some Imacon scanners and part of the price included having someone from Denmark calibrate the scanner down to the point of dealing with local variations in the Earth's magnetic field. The scanners were CCD, but were vulnerable to similar effects. Cool stuff if you're into it. And most manufacturers allow for it. There's the little FCC warning of "Must not generate interference and must accept interference" in the manual of most electronics.
Also, heat buildup can cause noise. Long exposures, even at low ISOs will introduce noise. Most Canon DSLRs have a custom function to deal with this. Rizzo, it might be off in your case. Think of your changing the sensor's ISO as turning up the gain on a mixer or an amplifier. You'll get a "louder" signal, along with all the other crap inherent to the system.
Next time you have a chance, really crank the mixer in the DJ booth (without having music on) and listen for the hiss. Unless you're Steve Dash, Gary Stewart, or Tony Andrews, there's gonna be a definable level of noise.
And that's it's for today's edition of Ask Dr. Stupid
. Tomorrow, we'll dissect an old 10D, strap a Peltier cooler to the sensor and have ISO 1600 images with the noise level of ISO 400.