I need some help finding the rattle on my bike, it does it really bad on tar and chip roads or other rough pavement, but makes no noise on blacktop. Its a metal on metal sort of clank sound that seems to be coming from the rear of the bike. I've checked all the bolts and everything seems to be tight. Wheel bearings are good and lubed.

One thing I noticed is that the Dura Ace cassette has some play on the spline on the wheel. I can move the gears up and down, I took the cassette off and made sure its on there correctly and retightened it, but the gears still have a bit of slop on them. This is on a set of Mavic ksyrium's, I pulled the rear wheel off another bike at our house with ksyrium's on it and the gears move up and down on that one too, is this normal for these wheels with a Dura Ace cassette?

I've run out of ideas on what to look for, any ideas on what to check next?