If it's dust, cleaning the lens with isopropyl alcohol is the best solution, but for a quick and easy fix sometimes compressed air will be enough. Purchase a can of compressed air and use the extension tube that is usually included. Open the tray door, insert the tube and spray air into the unit, moving the tube around. Use short burst of air. If you use long burst, the decompressed air gets cold and can cause some condensation. If the compressed air does not improve things, then manual cleaning of the lense or re-alignment is in order.