PS2 "clicking" problem

Does anyone know how to fix the problem I have with my ps2? When it plays games, (its ok with movies), about 50% of the time it starts to make a rather loud clicking sound, like its mechanical, but it can lead to temporary freezeups and affects gameplay. It is not game-specific, as I have tried several other games in it. It was fine until a couple weeks ago! I tried opening it up after someone suggested it was dirty, and I sprayed everything with compressed air, but it doesn't seem to have helped. Any help would be great! Thanks!

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