What's that noise?

There's an unidentified application on my laptop which keeps making a weird noise and its driving me batty.

I have 3 applications open: IE7, Windows Live Messenger and Skype. I know the noises that MLM and Skype make and it isn't any of those and I wasn't aware that IE7 made any noisesbut maybe I'm wrong. I've got antivirus and firewall stuff running, but I wasn't aware they made a noise either.

I would record the sound for you to hear, but as I don't know what action prompts the weird noise I can't because I don't know when it is going to happen. I've done a SpyBot and Ad-aware just in case the weird noise is linked to some background nasty that I've missed; I've checked the firewall to see if anyone is "poking me" and have done a virus scan to be on the safe side.

I still can't work it out, so I'm turning the sound off before I lose my marbles completely.

Right back to the packing ...