loud fan noise

my macbook over the past 6months has started becoming very loud, it is ok until i run any applications like safari, firefox, google chrome, any games, music, anything like that and my fan jumps to around 6200rpm and gets to a heat of 70degrees. i have checked the usual printing que problems and i have no que at all, i don't run lots of multiple applications and I'm using the latest operating system (lion) but i have no idea whats wrong, my CPU usage is only at around 50%. please help
thanks

Check Activity Monitor sort by CPU, it could be Spotlight trying to do its thing that keeps being stopping because the fans kick in.