Bicycle Mechanics - Another mystery noise

Sorry to just barge in with this, I've never posted in this section before, but I've had a mysterious noise coming from my road bike that's beginning to drive me nuts.
It sounds, and feels, a little like when it's shifting- except I'm not shifting, and it doesn't actually change gears, it just makes that sound like when the chain clicks into place. It's rare that I hear it- I might hear it an average of once every time I ride the bike- and doesn't occur with any consistent pattern, so it's hard to reproduce it for a bike mechanic to evaluate.
The bike is 17 years old with 8sp Campy index shifting. My first thought was that it might be the bottom bracket or a crack in the frame or the crank or something, but they all seem to check out when I look them over. It just came to me that it might be the rear derailleur- maybe a stuck pulley or something that's catching on the chain? Any other ideas?

My guess is that it'll keep up for a while then get more common to hear. Check chain stretch.

worn sprockets and old chain? possibly an old shifter cable? id agree with your conclusion that it's not the bottom bracket. bjoerges is right: as it gets worse it'll become more obvious.

It still shifts perfectly. The cassette looks okay, not too worn. I'll check the chain today. I am the second owner of this bike so I can't say for sure, but I don't think it was ridden a ton before I got it. I've probably put about 6-700 miles on it since i bought it last May. I just don't want to wait for the sound to get worse and have to try and find a lot of expensive replacement parts. But a chain isn't too hard, I just need a regular old 8sp chain, right?
On the other hand, my ultimate plan is just to ride this drivetrain into the ground and then replace everything with 11 speed. Maybe that'll just be happening sooner than I anticipated.

This is one of those times where it's extremely tough to diagnose over the 'net. It could be as simple as you need to center the chain better using the barrel adjuster. Or your derraileur hanger could be slightly tweaked, or your jockey pulleys may well be gritty and dragging. Or it could in fact be a worn chain or cassete. Or perhaps just a grunge build up in the chain or on the edges of the sprocket teeth. Or even you got some rust in the chain and now there are some stiff links. Or some combination of any or all of these things.

Thanks for the help, all. I got the chain checked today; it was nearing the end of its useful life, so I replaced it. I cleaned the drivetrain too. I didn't mention that I'd been trying a different lube that I'm pretty sure I'm never going to use again. There was soooo much gunk in that drivetrain, and I had only put maybe 60 miles on the bike since I last lubed it. I didn't hear the sound on the way home, so I'm guessing it was the combo of a worn chain on a dirty bike.