I cannot answer your question, but I have question to you, if you have many iBooks, what model is has least fan noise? and what most?
I have a few 12" 1.33GHz and 14" 1.42GHz iBooks. Not all of them work. I got a bunch of them "For Parts" and it seems like the 12" are worse for noise, although both are much quieter most of the time than my Titanium Powerbook 15". (The hard drive was a significant source of heat and noise in my Ti Powerbook 15: putting an msata+IDE adapter was a huge QoL improvement.)
It depends more on how you use them, though: I remember compiling big stuff (like KDE desktop env) on Gentoo Linux back in 2002 on my Pismo and that normally super-quiet laptop fan got really loud. I think these PowerBook G3 and iBook G4 laptop were not much for heavy computation even in their day: in the (admittedly beaten to death) automotive metaphor, they're dinky little economy cars, not monster trucks of computing.
Every Apple laptop I got after the Pismo was a disappointment one way or another. Nowadays, I only dabble in i-/power- books because they're cheap and, thanks to the software and information at macos9lives, finally starting to suck less.
I guess, if you are prepared for the hassle of replacing the 20+ year old disk when it fails, get the 14" iBook for low noise. I probably won't do it until mine fails.