My 2003 VW Jetta sometimes makes a chiming sound once I'm driving. What does this mean?

It's sometimes 2 chimes, sometimes 3 chimes. It usually happens a minute or two after I start driving, but sometimes it will happen after I've been driving for a while. When I hear the chimes, the radio/cd player usually dims in sound, if it's on, then goes back to normal after chimes are done. It says nothing about this in my manual, and it's driving me crazy trying to figure it out! Someone please help!!