Noctua makes their NF-A6x25 if you're not too worried about aesthetics (they don't have it in their black Chromax colorway).
While I have tended to like Noctuas over the years, m y experience with my last water cooling build made me sour on them quite a bit. A couple of years ago they started messing with the circuits on the PWM control on their newer fans in ways that make speed control not work if you split the pwm signal over a large number of fans. Because of this I have been avoiding them lately on principle.
I do think there are many options for silent or near silent fans. Heck, spin any fan slowly enough and you'll barely hear it. I doubt you'd need much airflow over this thing.
7 years ago I bought a couple of 80mm SilenX fans for a couple of HTPC builds, and they were silent enough that they were inaudible from TV viewing distances.
Not sure what the status of the company is today. Their webpage is still up, but surprisingly does not have TLS enabled, and it does say "Copyright 2011" everywhere, which is suspicious. They even list Circuit City and CompUSA as some of their retailers (lol), but they at least did make a really quiet 60mm model:
SilenX, The PC Silencing Specialists
silenx.com
I bet if I spent some time on Newegg sifting through their filters, I could find any number of quiet 60mm fans, and other 60mm fans I could make quiet by using a fan resistor, or just changing the wires to run them at 5v instead of 12v.
The Egg still has the 60mm version of the SilenX quiet fans, but ****, they cost more than Noctua. Noiseblocker is a name I haven't heard in a long time, and they also have those in 60mm.
(not the biggest fan of Newegg as a retailer these days, but they do still have class leading filter categories for their products)
As far as color goes, I'm thinking if one really wanted a black Noctua fan it would be fairly trivial to just use vinyl dye to dye it black. Since it is not paint, but rather pigment that is absorbed into the plastic, it doesn't leave a layer on the outside that is likely to destabilize the fan blades. I've used it on car parts, but I bet it could be used on anything made of plastic.
The 'north most' M.2 slots on most boards are above the top x16 PCIe slot, which is typically in the second expansion slot position. Something like a tower HSF would likely work if you mounted the fan on the CPU socket side as 'exhaust', i.e., blowing up toward the CPU socket. Would probably be precarious with air cooling, but should be workable for a water block.
Interesting. Is that a new configuration for Gen 5 boards? Because I am used to seeing the slot just below the first PCIe slot, meaining it is usually directly under the GPU heatsink.
