Does your computer ever make a weird sound for no apparent reason?

I am on a Mac and I have lots of odd little things running, doing backups, syncs, who knows what. Every so often my computer goes "boink!" with no error message or display and I wonder, hmm, I wonder what just happened?

I have a suspicion it's my hourly backup which is boinking because one file couldn't be backed up. But it might be finder saying that it was done copying all those files (oh wait, maybe that's a "bing" not a "boing") Or might it be something else totally?