Freehub shouldn't need internal re-lubing unless it's time for general hub maintenance, and that depends on mileage, riding conditions, and so forth. Maybe once a year or so? I always use a light film on the splines, then slide the cassette on and call it good. If there's muck on the inside end, it *might* effect the cassette spacing, but I think this is a long shot.
As to torque, 50Nm is *tight*, just about as hard as you can crank on the 12" wrench while holding the cassette with chainwhip. Hard to over-do it, unless you're stupid strong, or use a vice to hold the lockring tool or something. You should feel the teeth on splined side of the lockring kind of "ratchet" as you cinch the lockring down that last turn or so.
I also assume from what you've said that you've got the correct lockring, and all the correct spacers in the correct order, as per p. 77 here:
http://www.campagnolo.com/repository...s08_B_1007.pdf
If the cassette is tight, then the rattle noise has to be der./chain related.
Hope this helps...