iMac constantly ejecting, no CD in drive

Hi,

My late 2009 iMac (intel) is frequently trying to eject from the CD drive when there is nothing in there. The drive seems to be working well otherwise and it does not eject randomly when there is a CD inserted.

Often it will make the sound of trying to eject 20 or more times and then stop doing so.

Can anyone suggest why this is happening and what could be done to stop it?