I'd double check that the lock ring isn't bottoming out before compressing on the cassette--a 1 mm spacer may make the difference.
Last weekend I noticed the same thing on my nine speed--loose and noisy cassette cogs, though the lock ring was nominally tight. I didn't check torque before I tore it down to find the issue, but it wasn't easy to loosen. I verified proper spline alignment, no visible damage, and the lock ring had more than one full thread to go before bottoming out. When I torqued it to 40 N-m, it would feel tight to start, then loosen up after a test ride, though the lock ring stayed tight at spec torque. So I over-tightened it a couple more clicks and it's now quiet.
I searched this forum for a similar problem and didn't see anything. Now here it is, a week later.
I plan on trying a better cassette--the one on there is an off-brand, but it's been working fine for many thousands or miles. It's about time to change it anyway--I've been through quite a few chains on it. If that doesn't do it, I'll look for a new freehub body, but that at that point I'd probably want a new hub, or a new bike....