Not saying you have this... Are you using your own discs with stickon labels? If so, I know what the problem is.
Have you checked the chassis to see if the mounts are tight?
But if not and it really is annoying, NEC3550a is about $35 at Newegg. If it has been running a long time, it will wear the bearings. That is a very good reason NOT to leave a DVD in a player sitting on a menu (settops burn out this way - Barney hates DVD players)