Does it do that with all cd's? Reason I ask is it is not uncommon to run across cd's that were not made quite right and are off balance. Now, the cd's are not balanced to the level of car tires or anyting like that, but they are made to be within a certain spec.
If this is happening with just a cd here and there then I wouldn't worry about it. If it is doing this with every cd then it sounds like the internals of the drive are off and it needs to be repaired.