Have you checked the bottom of the post to make sure it is fully screwed on?
The issue you're having sounds similar to what I had happen to my KS post last year. When I took it out of my frame with the intention of sending it to be serviced I found that the bottom cap of the post had rattled loose. I tightened it up and it has been running well ever since.
Aaaargh this happened to me a few times. The first time it was a rattle I couldn't locate. It sounds like your rear axle is loose or maybe your DU bushing. Now when I hear that sound I know exactly what to do.
There's nothing better than an Orangina after cheating death with Digger.
