Look at the drive slot - is it smiling at you? If so, then the noise is probably coming from the disc rubbing again the plastic of the case. It is common for this area to become deformed, as the plastic is thin here. The primary cause seems to be carrying the macbook around open, squeezing down over the optical drive area.
If you still have AppleCare on this unit, I would take it in to an Apple Store or AASP to have the drive replaced.
If not, you can try to carefully work the plastic back to its normal shape using a wooden tongue depressor, or other fairly soft tool - DO NOT USE METAL! Only work the plastic, do not go in and deform the metal drive mechanism itself.