CD ROM/DVD drive spins up when no CD is inserted

A few times a day my CD-ROM drive will spin up even though there is no disc in the drive. I can hear it and feel the vibration through the case.

At first I thought it might be the fan, but it is definitely the same sound and feeling the drive makes when a DVD first goes in, or when I'm burning a CD.

It doesn't seem to be software related -- it's happened where there are no programs running that ever access the CD-ROM (e.g., iTunes is almost never running).

Any idea why this is happening? Or how to stop it?