Fan(s) running all the time

When running Mainstage, the fan(s) on my 17" Macbook Pro (2007) stay running whether I'm playing music or if it's just sitting there. The laptop is maxxed out with 3GB, and CPU usage is very low. If I exit the program, the fans stop after a minute or two. Everything seems to work OK, but I wouldn't think it should be running the fans like that.