GMC sierra radio randomly checks for CD's?

[misterhelpful]

The stereo unit in my 2001 GMC Sierra 2500HD will randomly switch to cd for a few seconds and then will switch back to fm when it realizes that there is no cd. Just tried putting a cd in right after it happened to see if it would start playing the cd if it happened again, but the unit won't take the cd. The two are related, because I switched cd's at least ten times the other day with no problems. Any ideas as to what could be causing this? Thanks!