CD player vibration from CD's

I was changing a cd and noticed a strong vibration in my Oppo BD83SE cd player. I have had quite a few cd players and have never noticed this kind of vibration before. I found it was not caused by all cd's. With some, the majority, there is very little or no vibration. I am assuming the cd's in question are not round. Anyone else have this happen with your players? I am especially interested in Oppo Players but anyone feel free to chime in.