There's nothing special about those ADDA PWM fans, other than bizare wiring/pinout scheme. The only thing I can think of is if the Noctuas were reporting rpm value / signal (through RPM SENSE out) that bothered BIOS, expecting higher value at IDLE. Still, not all 4-wire PWM fans are the same - I'm not referring to rpm range, or 3-wire vs. 4-wire variety (standard nowadays), but the algorithm (and chip) used for the fan circuit to communicate with BIOS. I wonder how that guy reporting success on Amazon UK got it working ...
I don't plan to replace any other fans - the rest of the fans are behaving well (for now), plus I'm on a tight budget, since I'm configuring this machine for a friend of mine, not for myself.
Thanks again for all your input, will report back when I swap the fans; I decided to pick some higher cfm fans, as the rpm rating of original ADDA (5000 rpm) got me thinking a bit. There's got to be a good reason why HP (or Delta) chose high rpm fans for this application - the space in Delta PSU is pretty crammed, plus there's quite a long air flow path, for the cool air to be pushed through by those two 80mm fans. Just a thought ...
Kind regards, Sebastian