Thanks everybody for the help. I think the way to go is to replace my PSU fan. Just to get this right, this should be a 120 mm x 120 mm 12 V fan? You can easyly find fans like this that only do 15 dB!
The A4000D fan is an 80mm fan. I bookmarked some really quiet fans at home, and will try to remember to post them here. I found the fans in this very forum somewhere, after digging around for a few hours.
slaapliedje
