This started a few years ago and doesn't happen very often. I normally have multiple applications running which always includes Firefox, in which I usually have at least 20 tabs open. Usually, I'm even not touching the keyboard or mouse. Usually I'm just sitting near the computer, not even actively using it. The sound associated with a single click of the mouse will occur, maybe once every minute or every few minutes. I don't know if it's associated with a particular web page or not, because I have so many open. I never see anything happen when I hear it. It's happening now, it's made the sound about 5 times in the last 20 minutes. Where could this be coming from and is there any way to find out? Is it possible that this is associated with a compromise of my system? Makes me wonder..