Joined: Jan 2008
Posts: 23,639
Likes: 1,106
From: Mans of NC & SW UT Desert
Bikes: 86 Katakura Silk, 87 Prologue X2, 88 Cimarron LE, 1975 Sekai 4000 Professional, 73 Paramount, plus more
I can't imagine a cassette turning on the freehub. What is a tiny amount? Real tiny = loose cassette, tighten it and you will be fine.
A quick stop at your favorite bike shop, they might do it for free for you, or at least take a quick look at it. Diagnosing a problem via the internet is difficult, with no pictures, even more difficult.
Good luck, hope it is something simple.