Mysterious audio alert

On random occasions I get an audio sound, like an alert, like a doorbell (ding-dong, high pitch to low pitch). It is a full sound, with slight echo and reverb. I don't know what's causing it.
I have gone through all sounds in the Sounds window in the Control Panel. It is none of those. I have modified Windows default to no sounds on all of the events. I have tried to turn off any sounds of email notification both on the Comcast side and the Live Mail.
Experiments to try and cause it by sending, opening, receiving email did not cause sound. I have turned off sound on all games. I have tried watching Task Manager to try and catch activity in processes when a sound occurs, but have not seen anything.
It is starting to drive me nuts.
What can I do to find the source of this sound that just happens from time to time? It is sometimes a couple of times a minute, sometimes only a few per hour. The only task running pretty much all the time is BackBlaze, doing backups of changed files. I have tried changing and deleting files to see if I can cause the sound. Nothing!
I do a fair amount of recording of my piano work using Audacity, but I haven't heard the sound being recorded along with my music.
Any clues?

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