Greetings,
   First the moan.  This is normal for the XJ40 as the steering rack ages.
Check your fluid levels.  If they are all OK then you should be OK on the
steering rack.  If the level is down a bit then you may want to top it off
with some sort of stop leak with lubricate in it. That will generally ease
the problem.  Watch it, however for it may be a sign the rack is aging
towards replacement. (common with these cars)
   The rattle is a bit different, however.  My first thought is that the
tyres are out of balance. This would cause the wheels to "bounce" at certain
speeds causing the washboard sound. Basically it creates an out of round
situation. I would check with the local tyre guy.  The other possible
explanation is the poly washers in either side of the wing where the shock
comes through the wing on the top mount may have deteriorated to the point
they are starting to chatter at certain speeds.  This is common and
eventually, the noise will turn into a "thump" when you go over bumps.  The
spacers are easy to replace.  The 3rd possible is the shocks have
deteriorated to the point the wheels bounce at certain speeds.

Cheers
Webserve