Check all of the cable connections on the back of your CD player. Secure any loose connections you come across.
Clean your CD player's inputs by pulling out the wires and spraying compressed air into the inputs; this will get rid of any dust or dirt that may be affecting the sound quality.
Clean the CD player's contacts, situated on the player and the receiver, using contact cleaner.
Check CDs are not scratched. Clean by dampening a lint-free cloth with some scratch-remover fluid and wiping the readable side of your CD from the center to the outer edge in a linear motion.
Clean the laser lens by applying some scratch-remover fluid to a lint-free cloth and gently wiping.
Check that the transportation lock -- usually a screw situated at the bottom of the unit -- is not engaged. Unscrew the lock using a screwdriver to disengage it.
Open the CD draw on your stereo. Check the draw-loading belt to make sure that it is still tight; if it is visibly loose, take the CD player to an audio repair store and have an expert replace it.
Check for broken or loose CD draw gears; again if you find any, these can be replaced by an audio repair store.
Check for any foreign objects in the draw area such as pet hair. Clean the draw, and the area behind it, using a clean lint-free cloth.