I have had both Winterhoff (which I prefer) and Alko which are also good.
I see lots about them being noisey, and all the attempts made to clean
and fettle them.
My current caravan has an Alko, I cleaned the towball and the friction pads
in the hitch a wipe out first then a rinse and wipe with brake cleaner.
I then hitched up and it groaned all the way to Slapton, getting embarassingly
loud on the site. Leaving I cleaned the ball and hitch the same way but left it
overnight, It was as quiet as a mouse on the way home.
I put this down to the cleaning removing ... wait for it .... the atmospheric moisture.
I decided this when the Winterhoff on my Puck L 230GT did the same and to save
embarassment on the site I gave it a quick wipe over with a clean damp cloth, silence
So there you are.
Don't take it for granted it is the ball and hitch, I remember someone oiling their
spare wheel carrier and solving the noise problem.

WE had the noise until I discussed it on the other forum and after the cleaning job similar to yours, it worked a treat on my Alko.

Ours groans with a brand new Alko hitch (replaced last year when an over enthusiastic breakdown truck broke the last one). I quite like the groaning as it reminds me that she is still there (you wouldn't know she was there otherwise).

Our new Troll became very groany after a few months. It was noticeably worse going left rather than right. When I tried to take out the friction pads for cleaning one of them appeared to be stuck. Eventually I discovered that there is a small 'O' ring seal in the tube that the friction pad goes through. This was unseated and jammed around the shaft of the friction pad. I believe the jammed shaft was causing the groaning which has now stopped after replacing the o ring.

I don't have a problem, all I can hear is the roar of the Mini's Cooper S engine! The Puck and Mini have just come back from being serviced so everything has been greased up and wheel bearings and brakes checked. Went to pick it up this morning it seems to run smoother than ever now. In fact I almost forgot it was there on the way home and suddenly realised I was going too fast (oops).