Could I replace a stock PSU fan with a more quiet one?

"I have done this. It works.

If you're nervous around capacitors, be aware that they can and will store residual mains power. Practice basic safety: Unplug the psu, ground yourself against the case. Snip the wires, strip them, and use a 'decent' wire splicing method. wrapping them with electrical tape is not a good idea. Solder + heat shrink, small wirenuts, ""B-connectors"", etc.

I've been hit by the caps, you feel it, but it wasn't horrible. As always, proceed at your own risk, but I've done this plenty of times.

I wouldn't recommend fiddling with the rest of the components, but just a simple wiresnip on the fan leads is pretty painless.

Caveat: Some PSU's run their fan on 12V, some on 5V. If you have a multimeter, check and make sure you pick a fan rated for those speeds. Most 5V 'can' run on 12V, but it's not recommended."

"It's not problem replace PSU fan, but you loss warranty.

I prefer Noctua Fans"

"Some expensive PSU (like Seasonics platinum) requires fan (Hong Hua fan) replacement. It is not only a problem for cheap PSU. Lets describe how to replace Hong Hua HA13525H12F-Z with proper fan.

You should select fan carefully:

Find your stock fan product number (like HA13525H12F-Z).
Find characteristic curve for this fan.
Characteristic curve for Hong Hua is not available. You should select another PSU with same characteristics and same amount of heat (old Seasonic platinum platform with Sanyo Denki 9S1212H403 fans).
Open Sanyo Denki 9S1212H401 (9S1212H403 has same characteristics) characteristic curve.

Now you should select fan with characteristic curve above or equals to Sanyo (12v) curve. My solution is Noctua NF-A14 industrialPPC-2000.
Ask Noctua to send you a characteristic curve for selected fan.

You can see that curves are about the same, difference is low.

Buy your fan. Use any buck boost step down converter like LM2596 to test your fan. Starting voltage should be low <= 5.0v. My Noctua industrial fan has about 4.85v starting voltage, it is fine.
Shutdown your computer, remove PSU from computer case and wait for 10 minutes. All your capacitors will be discharged during this time.
Open PSU, remove Hong Hua, drill additional holes, solder Noctua directly to PSU mobo. It is better not to use fan pin headers, because Noctua motor starting current is high. Soldering is more reliable.
Test how your fan controller is working using auto transformer on 100-250v with different load. You can use regular bulbs as a load.
You can attach additional temperature sensors on radiators during tests. Sensors under load can be under high voltage. Just do not touch them and everything will be fine.
Remove sensors and install PSU back to the computer case. Enjoy.

Results:

Hong Hua HA13525H12F-Z - 51.7 dbA
Noctua NF-A14 industrialPPC-2000 - 31.5 dbA

I think it is a safe solution for Seasonic platinum/titanium <= 850w/1000w. But of course it is your own risk, nobody is in charge."