Random delete noise every once in a while

On the MacBook Pro I'm using, every once in a while I hear the sound effect that plays when you delete something...I'm not sure what it's called (a sort of poofing noise). It goes away when I uncheck "Play user interface sound effects" in preferences, so I'm guessing it's not a bug. Any way to check what it's coming from? I have Firefox open almost all the time, but other applications change and the sound doesn't go away. Any ideas? Thanks!

 

I figured that out, but I've still had no luck identifying it. I've tried using the console to see if any commands come up right after the sound, but nothing. I thought there was something- a com.microsoft.Word[529] thing, but it didn't reappear after subsequent noises. It's never immediately after an action I take, and most of the time it happens when I'm not pressing any keys. I'm rather stumped- this isn't my computer; it's a school computer, so I thought it might be monitoring software or something. But I closed the firewall as much as reasonably possible and the delete poof still persists...It's kinda creeping me out.