Top Loading Transport Question

I own a CEC 5100 transport and every once in a while I'll get a cd that makes mechanical scuffing sound while spinning almost like the cd is warped and is rubbing the body of the player. When the cd is checked though it appears to be flat. Has anybody else experienced this problem?
I have one cd (unfortunately a favorite) that makes a scuffing sound in both a portable cd player and a Parasound CBT2000. It plays ok and does not seem to damage anything so I have not worried much about it.
I've had that happen and found it correlates with the kind of markings/decoration on the top of the disc (a lot of paint, etc.)..other than the annoying sound, there were no ill effects.
The pioneer stable platter avoids the effect altogether I believe, since the disc is placed upside down.