What computer is it??? laptop / desktop??? do you have any "hotkey" software running (especially on laptops).
The only thing i can think of is that once in a while, while you are scrolling or idling, the computer tries to launch or do something. A while ago i had shortcut keys assigned and since most people tend to use the keyboard a lot (cntl, alt, i, c, and a couple more) some shortcut keys may get triggered. Resulting in the "dong" because it cant execute the shortcut from within the game....
If this is not the problem.... then we will think of the next possibity...
