Twice now in the past few days my Mac has chimed every 10 seconds continuously while usage for an extended period of time. It's that two-tone chime that would come up with an error or popup sound, except I couldn't find any program telling me anything. I couldn't tell if it was coming from any specific program. The first time it happened i had to mute the speakers, and a few hours later it was gone. The second time it did it for about 10 minutes then went away. Am I just going nuts?