Be sure to store the screws sorted by assembly step so you do not mix them up when putting things together again.
This time I found that the fan is attached with two small black screws only on the left side so I was able to completely remove it from the unit which gave me opportunity to entirely blast the heatsink clean from the inside with compressed air.
The fan itself I first blasted clean with compressed air, then cleaned it with electronics cleaner (Reiniger 601 from Kontakt Chemie) and then soaked it in Ballistol resin free oil (I really love that stuff). Finally I removed all excessive oil and cleaned with air again to avoid any oil spill in the laptop.
To my surprise the thing now runs as silent as on day one again and I tested it with various fan control programs to confirm. Very nice. Lets hope the Mac makes it another 4 years!
So it appears the noise from PC fans does not always come from mechanical wear and tear as I expected but rather from running dry which might be fixable!