That's possible, I suppose. If there is a loose screw inside the player, that might cause the behavior you're seeing. Or perhaps a completely loose drive tray assembly (rubber mounts came out). I wouldn't rule out external connections in the car wiring though. Definitely check the external wiring as well.

Also, you said you've had it for two years? Now we have to ask: is it a Mark1 or a Mark2 player?

If it's a Mark1, possible problems are: Loose drive mounts, disk drive cable rubbing on a motherboard jumper, audio riser on back of motherboard loose.

Also, the docking connector on the Mark1 needs to be cleaned regularly (alcohol and q-tips) to prevent it from making dodgy connections. It could be that simple.