....snip...

Check if there is a tiny microswitch somewhere. I've recently seen a bluray
drive which did something similar because a microswitch, which was activated
by the movement of the carriage, got stuck. The mechanism used this to set
its initial alignment and so the carriage thought it was "fully forward so
need to move back" when it was actually fully back with the motor squealing
because it could not move the carriage any further.

As others have hinted, dust in/on an optical sensor or other sensor-knacking
could be causing the same effect.

Paul DS.