...as if there was a rubber seal twisted no movement in the freehub...
jags, that would be my guess, as well.
Have you tried taking it apart? If the hub has cartridge bearings, then the freewheel body is often held on by the axle spacers, which in turn are retained by little teeny-tiny grub screws that take a 2.5mm allen. I have a set of very nice Bontrager-clone hubs like this. No replacement parts are available, but in the meantime, it is one of he nicest cassette hubs designs I have seen.
And yes, the seals can get dislodged, causing a continuous squeak. It is amazing how long it can last, given the wear that must be taking place.
Have you tried lubing the joint between the right-hand hub flange and freehub? You'd have to remove the cassette. Also, a surprising number of non-Shimano freehubs have handy little oil ports in the freehub body that become visible oce the cassette is removed, a drop of thick oil there (i.e. Phil Tenacious) wouldn't hurt.
Best,
Dan.