If you don't find an error sound in Control Panel, it's possible you're hearing an outside interference.

If it's an external source, about the best hope is that you can find a "simultaneous event" that you can blame it on. (Being accurate is difficult, but sometimes it's sufficient just being able to asssign the blame to something likely enough that you don't feel guilty about ignoring it?)

One of our desktops in particular is subject to repeated "dit dut dit dit duh" noises that it took us quite a while to find were caused by a cell phone being "pinged" to keep track of which cell tower it was closest to. The "solution" was for her to keep the cell phone in her purse where it belongs, but that's only "variably successful" since she hardly ever knows where the $@#!%! thing is in order to answer calls.

John