Random Thunder Noise but No Virus Found?

UPDATED: 12/26/08 The annoying thunderstorm sound is the weather channel gadget that sits in your system tray next to your clock on the lower right of your screen.  I actually need to confirm this with a friend that uses it but it makes perfect sense.  I didn’t realize that one of the users on my parent’s computer was using the weather channel instead of weatherbug.  The account is usually open when other people are on it.  It’s confirmed.

Open question on this one.  My parents are getting a random thunder noise on their computer.  They say it is completely random.  After much searching I found something similar but they didn’t have the files that were listed on the internet.

I uninstalled AVG which I have raved about and installed Avira?  It was that one or another one.  Whichever one was ranked as the best this year.  Still no viruses found.  I’ll be up for christmas and will have to listen to it to figure out what they are talking about.  Very weird occurance.