take off the cogs, [2 chain whips] then look for the allen hex sleeve bolt that holds
the freehub sub assembly on,
once you removed it, drop it in a cup of solvent.
[cogs off it won't have to be that big a cup]
spin and blow it dry with compressed air.
lubricate again .. 90 wt gear oil .. Phil tenacious etc.
if you cant separate the freehub. try to plug the other side so it
will itself fill be the container of solvent,
to soak out the old oils and the grit it holds inside.