I'm based in Prague, Czech Republic so Holland is reachable for me (will send Paul PM). I also found some company in Germany that claims to be able to repair K4B (
link). Just the price 4690 EUR is quite high.
And yes I mean differential. I have standard one without limited slip feature (at least there is no sign of special oil on it). Do you think it's worth of opening it? I mean are there some spare gear parts still available to reduce the noise?
Filip