I would say this is most likely an issue with the disc drive, or any Wii components relating to the reading of discs. I've got a few questions for you:
Have you damaged your Wii in any way recently, such as dropping something heavy on it, or dropped the Wii itself? Have you moved the Wii while it was on with a game inside? How old is the Wii? Does the Wii sit vertical, or is it laid horizontal on it's side?