I've been trying to track down a small but annoying squeak coming from the back of my bike and I'm not having a lot of luck. I'm now focusing on the cassette. There's a small amount (1mm or so) play if I pull on the cassette. I've removed the cassette and the freewheel seems solid, it's as if the cassette isn't sitting quite square on the freewheel and rocks slightly. With the lockring tightened down play is reduced to 1mm or so, but it's still there. Is there anything I can do about this and could this be the source of my squeak?

What hub and cassette are you using? And has the squeak only just started or been there from fitting?

It's a DMR Revolver hub and a Deore cassette. I'm not really sure when the squeak began but I did notice it not long after fitting the cassette (I had a SRAM unit fitted prior to that), I don't think it's a problem, just annoying really but I'd like to get it sorted.

I had some play in mine 4 weeks ago. It started being a problem changing gear.
Turned out to the free ride hub and bearings. Mavic hubs have an odd set up I'm told, so it was originally diagnosed as something else.