It is more than likely a piece of that greyish inner bezel (behind the clear pinstriped bezel on the front). My Indigo is literally falling to bits because that brittle plastic is just disintegrating

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That seems to be a common issue with these machines, Classic Mac 500 posted a while back that his machine has the same issue. Strangely though the tray loaders don't seem to suffer it, I'd say that either Apple switched to a cheaper plastic with the slot loaders, or its caused by the excessive interior temperature that the slot loaders run at (since they have no fan)