After some jamming, I noticed my guitar makes a clicky sound every time I place my finger on some open string (it can only be heard with an amp connected, maybe it's too soft for it to be heard unplugged). I first figured it was hitting itself with some fret, probably because of bad neck calibration, but it keeps clicking even if I touch it very gently and almost not move it at all. All I could find over the internet related to this is sounds that are produced while lifting the finger, not placing it...
Some introductory facts:
- The strings of an electric guitar must conduct electricity. That's actually how they work. When the string vibrates over the pickup, a very small current flows in the strings at the same frequency that the string is vibrating. Then that current causes another current in the pickups, again at the same frequency, and the pickup current is amplified to make the sound you hear. If you put nylon strings (that don't conduct electricity) on an electric guitar, you won't hear anything when you play it.
- The strings of an electric guitar are grounded via the bridge. The bridge conducts electricity and inside the body of the guitar there should be a wire that connects the bridge to the ground - usually the casing of one of the pots. This is to reduce the effects of the strings basically acting like radio antennas.
So, with that in mind, here's what's happening:
- When you're not touching any of the grounded parts of the guitar (bridge, strings, jack plate, possibly other parts), the strings are basically radio antennas. They are picking up radio waves that are then picked up by the pickups and amplified. Usually these radio waves are fairly weak and most (but not all) of the current induced by the radio waves bleeds off to ground and is not amplified.
- When you touch any grounded part of the guitar, the current induced in the strings by the radio waves partly bleeds to ground and partly bleeds into your body. This dramatically reduces the amount of current that is left in the strings and picked up by the pickups. So touching a grounded part of the guitar reduces the (usually quiet) buzz that you get with an electric guitar.
- Right at the moment of touching it, you're basically closing a circuit. Usually when circuits close there's a very rapid change in the flow of current, and that change is so rapid that it sends a pulse through the relevant conductors. That pulse is picked up by the pickups and is amplified as a pop.