It's designed (or should be) to be used safely and dependably in the "hottest spots on Earth," so to speak. Therefore it's fair to wonder if a (very) noisy fan is really needed at normal room temperature (say max. 22C or 72F).
I've seen the question often and it seemed often (unofficially) claimed it is quite ok to have some silent cooling for it in these lower temps.
If I was gonna keep my II, it's what I would do at some point, as even the silent fan version is kinda loud (yet the process to install a larger one is not that attractive).
Of course, FAS is not gonna recommend this, but if you're prepared to honestly take the blame and buy a whole new motherboard if it fails, I'd go ahead and install a variable speed fan or something.
There's of course also CPU differences. Clearly, some of these hardly need a fan at all, and will only have it spin up when needed.
In Europe, at least for the II, it seems G66 is allowed to replace fans with a "silent" version if the user so prefers (which often still is not nearly silent, esp. the smaller ones). As these usually have less CFM, it indicates it is ok to a point.