I have lost two Liteon 20a1p drives to the same problem.
It seems related to the electrical motor that drives the cd/dvd.
If i shake the drive i hear something, like there's loose sand inside the drive.
So i open it and traced the sound to the drive motor and after opening it with the help of a screwdriver it seems there is something loose in the upper compartment of the motor (black part).
Some pics attached.
It seems like planned obsolescence to me :a http://en.wikipedia.org/wiki/Planned_obsolescence
I have a samsung cd-rw and a btc cd-rom with the same issue.