PS3 Slim (120GB) not reading disk right away!?

help! when i insert a game disk it doesn't appear on the XMB right away, it makes a whirring sound, and after a few seconds it makes the insert and eject sound, and then whirring sound again, what i do is put in different kinds of games and IT WILL WORK AGAIN after a few ejects.. the weird thing is after over night it will do the same thing! is this normal?