So it seems I picked one of those Seasonic Platinum PSU, which develop coil whine. Under load it's really loud now.
At first I was considering a Corsair CM series, but thought to first try the bequiet PSU (L8 430W) - which was bought originally for this build - with a different fan. It's stock fan used to be quiet for the first 20-30 minutes only, as soon as the temps were rising it would ramp up and produce whining from the bearings. Still had a NF-S12A around, which now is residing inside this power supply:
I routed the fan wire outside the PSU enclosure and it's now daisy chained to the CPU fan. The CPU fan connector is now driving 3 fans: CPU, back exhaust and PSU.
As I had the unit opened anyway I also put one of the kaze master temp sensors on bigger heat sink inside the PSU.
Did a few experiments and found, that this PSU needs proper active cooling with fresh air. It gets too hot if it's turned with the fan facing inside (like in the photo), thus it is turned upside down now.
It switches off at ca. 70°. If the fan is running on a Low-Noise Adaptor, the PSU can get too hot und heavy load.
It's running now at the same speeds as the back exhaust and stays cool enough even under the worst artificial loads. At gaming it goes up to ca. 60°, idle <40°.