It’s probably the CRT, but there’s a very easy way to be certain.
Turn the monitor off while recording, does the problem go away? If it does, then you can either always turn the monitor off, or buy an LCD monitor (my LCD monitor doesn’t get picked up by anything, I’m pretty sure this is the case with all LCD monitors).
If that doesn’t fix the problem then you’re picking up interference elsewhere.
Glass monitors have very powerful electromagnets a foot or so behind the screen. Anything sensitive to stray magnetic fields, like, oh, a guitar pickup, is fare game. The problem is so powerful that many times two glass monitors placed next to each other will shimmer and shake as the two operating magnetic fields get into each other by accident.
The CPU generates interference, too, but that trash ends up mostly affecting portable radios and is nothing compared to the intense problems that monitors have.
Funny story. Back in college, I used to have a small fan sitting nearby my monitor blowing stale hot dorm room air around the room. And I noticed that I would start getting motion sickness just sitting there working on my computer. After a few weeks of this, the weather calmed down and suddenly I was able to work on my computer again.
I was, as they say, baffled.
But then I realized that whenever the fan was on, the screen on the monitor would shake ever so slightly (it was just barely noticeable at the corner of the screen). The rotating coil in the fan’s motor was making the screen shimmer and thus making me sick.
Just goes to show you can never ignore the power of electro-magnetics.
Similar story to alatham - I used to have a DEC VT100 terminal on my desk - next to it I sat one of those magnetic driven fishes in a plastic aquarium.
I kept reporting a rippling screen - every time the engineer called I moved the fish tank to give him room to work - hence no shimmer while he was there. He goes, tank with moving manet goes back - screen shimmers again. Took me weeks to figure it out - and did I feel so stupid …
Guess what! I tried recording with the monitor off. I get the same interference. I’m constructing a “groaner” box: a box lined on the inside with foam to cut out the fan noise of the CPU, and lined with aluminum foil on the outside to eliminate the interference.
Wireless stuff like that broadcasts at frequencies that are much too high to affect a guitar pickup. Anything that got picked up would be filtered out as soon as it hits the guitar cable.
That said, if you put your cell phone on top of your guitar pickup, you might hear something, but you’ll have to get really close I think.
Florescent lights and dimmer switches are a different matter. I also find that my CPU case seems to put out something that my bass’s pickups don’t like either. I have to sit facing away from the case to get a silent recording. It just occurred to me that it may also be my stereo receiver. I’ll have to test later at home.
My personal hate at the moment is a steady ticking noise on my amp/speakers (connected to the sound card, but there even if listening to something else) that goes with screen refreshes on my TFT monitor - I think it’s some thing to do with grounding and my graphics card, so I’m going to try a bunch of ferrite cores on the audio cable to put some inductance in - I’ve already got a filter in the mains power to the amp.
Mobile phones have a hidden problem. If you have one on vibrate and it rings while it’s sitting on top of a glass monitor, it can put a semi-permanent color blotch on the monitor. All those conditions have to line up. Vibrate mode on a mobile runs a small, unshielded electric motor and it can produce an enormous electromagnetic field for a short time.
You should find your interference by setting the computer so you can hear live what you’re doing, turn it all the way up and wave the guitar around until you find the trash. we have a really tiny lamp dimmer next to one of the audio consoles and the console would fail only for those people who liked dim light.
I used to have an unshielded guitar pickup without the guitar. It was great for finding hum and buzz problems.
The problems can be seriously convoluted. My entertainment system would hum until I disconnected the roof antenna (different grounds). Get one of those three-light AC power testers (that I can’t find a web site for) and plug it into all your wall outlets. Make sure the right two lights come on (they’re marked). If only one light comes you you may be missing the safety ground inside the wall and that’s a serious problem.