somethings try to access it , the question is what ? The answer hopefully lies in filemon from
http://www.sysinternals.com
download it, install it, run it, deselect all the drives except for the CD drive (in filemon) . watch for a program trying to access it, then find that program. If its not obvious what it is, come on back with the info it gave you..
it could be something simple like the indexer, or some tool that watches the drive for media,.. or the disk inserted switch might be faulty, see if the CD's disk letter "eject" menu item is greyed out or not