Rubber dampener, where does this go?

Okay, I found this lying in the bottom of my Intel Mac Pro case. I opened the case to look at the optical drive, it started rattling something fierce when a disc is spinning away in it all of a sudden.

Lo and behold, it must have something to do with this black rubber absorber/dampener falling out. It is sticky on one side so it must go somewhere on or near the optical drive. My question is: Where?

I've looked all over and can't find where it might have been. So, if anyone does know or, remembers seeing it somewhere on their mac please let me know.