The noctua fans caused some noise in the left channel. I think it was caused by the taco pulse signal. So I rearranged the cables making sure the yellow wire from the fan runs far away from the amp/input sections.
Also decided to measure the variable voltage fan connectors. Voltage seems regulated by the power supply temperature. When the amp is cold the fans run at 8,5V, increasing to about 22V when the amp is very hot. I actually think the stock 24V fans are decent -but you can definitely hear them at 8.5V
My noctua fans are 12V so I decide to mount two pots to regulate the stock fan voltage.
First pot regulates the two fans cooling the amp heatzink while the second pot regulates the fans in the power supply region (right side inside amp).
I tuned the pots so the fans runs slow and SILENT when the amp is idle. When the amp works hard the fans runs kind of loud but at high volume output I don't notice the fan noise.
So now I'm very pleased with my slightly modified amp