I have a 13" MBP which I bought a few months ago. Everything works fine, battery life is wonderful, it couldn't be better. A few weeks ago it began "chiming" about every 20 minutes. One single tone - not a beep - more like the chime from a tuning fork. No message is displayed on the screen when it occurs. Sometimes it happens when I'm using the machine (normally web browsing). Other times it chimes when the screen is closed down and I'm re-charging. It doesn't happen when I start up.....I've even checked to make sure there are no other browsing screens opened in the background. Any ideas??