Harman Kardon Scratch/Hiss Sound

I have a Harman Kardon FL8550 5 CD changer that plays about 40% of my CD's fine and the other 60%, plays them with a pronounced hiss or scratchy sound in the background. There is no rhyme or reason which discs play fine and which discs play with a hiss or scratch. Older, newer, classical, rock, makes no difference on outcome. I have tried cleaning the lens with a dry cd lens cleaning disc that has brushes on it and there is no difference. Any ideas on what's going on?