Compared to other all-in-ones that I've used, I think Apple is doing the best when it comes to fan noises overall on all of the Mac computer line up. I could only imagine how loud will iMac Pro will be since it's talking about multiple fan blades and 70% better cooling capacity, despite Apple's statement which says it will run just as quiet. We will just have to see, but on my 2017 iMac I measured at around equal fan noise compare to my outgoing 2015 27inch iMac, both core i5.
It seems Apple may have more than 1 vendor for the fans or that it's just a fan to fan variations. I had returned my i7 for an i5 due the cooling fan spinning up while doing previews in Adobe LR and my new iMac just arrived yesterday. The very first thing I noticed was that it was quieter. The idle speed of my i7 was in the low 1200s while my new iMac with the i5 idles in the high 1190s. It's too small of a rpm variation to account for the noise but I can definitely hear the difference. The new computer's fan is maybe 25% quieter and I honestly had to put my ears up to the intakes near the speakers to hear it. My older i7 that went back was clearly audible and even had a varying "clicking" noise that quickly went away when I took it out of the box initially.