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Help tracking down a random beep

On my Debian Jessie system, for several hours today I've been hearing a beep 
(through my audio system / speakers / headphones) something like the beep that 
used to come out of the PC speaker (on older computers -- I don't think I even 
have that speaker connected on this computer).

The beep occurs at fairly random intervals, varying from about 5 seconds and 
going up to as much as 5 minutes or more.

I've tried looking at a lot of the logs, but so far I haven't found any log 
that tells me anything about that beep (the logs I've looked at include 
syslog, messages, Xorg, kern, daemon, wtmp (??).

Does somebody here have some suggestions on trying to track down the source, 
preferably without having to shut down any applications or the computer?