I'm really stumped about this one. Before on OSX 10.5 and now on OSX 10.6, my Mac occasionally beeps at me. It's a split-second "bee-bee-beep" that sounds vaguely similar to the push-to-talk beep you hear on Sprint/Nextel PTT commercials. I haven't been able to isolate what's running when it happens or what happens before it happens. Possible culprits are Quicksilver, Firefox, DropBox, Evernote helper, TrueCrypt, Wally.
PS I'm starting to wonder if Greasemonkey for Firefox could be the culprit.