It is the rubber isolator that sits under the airbox (airfilter housing). Has anyone removed the airbox at some point? What happens is the isolator gets stuck to the air box and gets pulled off the stud it sits on on the framerail. It then falls out of its locator cup (it looks like a hip socket) on the underside of the airbox. If you don't know it's supposed to be there you'll not notice it missing.
Take the airbox out, use a flashlight and look on the framerail below the location of the airbox and you'll see a (threaded) stud on the framerail. The hole on the flat side of the isolator is for locating the isolator on the frame, then the airbox sits on top of the isolator. So to reinstall it, just pull the airbox out, relocate the isolator and reinstall the airbox.
It's circled item No. 9 in this picture, which indicates its locaton on the airbox.
Last edited by Efthreeoh; 12-07-2012 at 10:24 AM.