Error code 6 & error code 4

my cd player won't play my cd's, and these codes come up. What do they mean and how do i fix them When I try to play even hard copy cds, error 3 sometimes shows up. Other times the cd will play but there will be a rythmic scratching sound louder than the music itself. Almost like every time the cd goes around it makes a scraping sound. I've tried multiple hard copy cds and it does the same thing with all of them. Any suggestions?