Mac beeping every minute, exactly

My Mac (a MacBook Air, though I doubt that's relevant) has for the last few weeks been beeping (giving the alert noise, that is) every minute, exactly. It is getting to be extremely annoying, and I cannot find the cause. Nothing is showing up in the Console log on a minute-by-minute basis, and there's no crontab entry to run every minute. I've tried shutting down various programs and background processes to see if I can eliminate the source, but that hasn't turned up anything yet.

Once it starts I can't find any way to stop it, but apparently it is not 100% consistent, because (oddly enough!) it is not running right now. Nonetheless, it happens often enough that in retrospect it seems like it happens all the time. The only work-around I've found is to turn the alert volume all the way down, but of course that's not a solution since there are times when there are real alerts, and I want to hear those.

What could cause a Mac to beep every minute?

SoundBunny is a new app that I have just learned about. There is a free demo available which will work for 15 days (I think).

The app window will show you every app which is makes a sound. That might help you track it down.

You can also tell it to ignore apps which you have ruled out, which should help you narrow things down a but too.

I hope that helps.

(n.b. I am not related to this app except as a customer. The purpose of SoundBunny is to let you set the volume for each app, but I thought it might help with this as well.)