I have two new Stern machines that I wanted to upgrade the power supply fans. On one, I will use Pinmonk's internal fan kit. It's very nice and plug & play and is a simply process to install for most people but requires the power supply to be opened. On the other, I printed (Scott Moschella's 3D model) fan adapter and purchased a great ball-bearing fan on eBay that run almost silent at 12V. This external approach is a bolt-on... requires no disassembly of anything. The adapter cost me $12 to get printed and shipped. The FAN was about $6 each.

The Pinmonk may be the cleanest for technically inclined, the 3D adapter approach may be easier for mere mortals.

The 3D adapter file from Scott Moschella:
https://www.thingiverse.com/thing:4844962

The Fan:
ebay.com link: itm

3DFAN (resized).JPGFANa (resized).JPG

The german Papst fan pictured above is also running whisper quiet, mostly I think because its a 30V fan that works all the way down to 12V! Candidly I can't hear it running while I have my ear a foot away from it. I can't compare how much air is being moved between the Papst and the Noctura fan, but my machine seems to run cool and the Stern PS fan hasn't kicked on at all...