As long as it's not scratching any discs I'd venture a guess that it may be the belt that moves the eye, that could be slipping, or a gear isn't catching properly, if you have any warranties left, I'd hurry and cash in on them, otherwise you'll be looking at getting a new PS2 or paying for repairs. I could be off on this guess by a bit though, I've never heard the noise, so I can't be certain.