Bicycle Mechanics - Shimano 105 brake caliper rattle - qr mechanism

My CAAD9 is perfectly silent - except for a very annoying rattle coming from a "shim" on the wire side of the quick release on the rear brake caliper.

See the picture.

Does anyone know how to rectify this rattle?
The "shim" or metal ring seems to be loose, when compared to my DA and Ultegra SL brakes. It makes a rather annoying tingling, ringing noise going over even the slightest road imperfections.

http://i100.photobucket.com/albums/m24/dagjm/IMG_6652.jpg

Thanks for any help :-)

It looks like a washer but the Shimano parts diagram doesn't show a washer at that location.

My 105s have this piece. I notice that when the QR is open, there's the slightest bit of "play" in it, but it's pretty snug when the QR is closed. By coincidence, I rode a couple miles with them open (broke a spoke) a few days ago, but I honestly don't remember hearing any rattle from them.

This one is pretty loose and the sound is very much pronounced.
It definitely differs from any other Shimano brakes I have been using - DA7900, Ultegra 6700, Shimano 105/5600.

It's almost like riding with loose change in one's jersey pocket or hearing the rattle from a set of keys.

The washer is loose to the touch - whereas on my other brakes it is not.

And yes, Bezalel, I also checked the Shimano tech document but could not see an exploded view of the QR mechanism in the PDF.
No part of the quick release mechanism is described by Shimano afaik..

You could put some heavy lube on it, should stop it rattling without messing up function.