Apparently there are two fan manufacturers that Sony used - Delta and Nidec (this one). My PS4 had a Delta one in now it has the Nidec and the difference (coupled with the new thermal paste) is remarkable!
The operation to change the fan means pretty much completely disassembling the PS4 (see the image) but I was at the point where something needed to be done. There's a video on YouTube that shows how to do it, step-by-step and, if you take your time, it's actually quite a fun thing to do.
For me, if you have a really noisy Pro and you're reasonably tech savvy, this is a no-brainer! Do it, you won't regret it. I'm delighted with the result.