I prefer a gentle consistent "woosh" and no humming or vibrating type noises. More fans make this more difficult. A few big 120mm or 140mm even better is all most systems need and those can be dialed in for sound characteristics while still moving enough air. I try for a very quiet system at idle, at gaming things can ramp up but I let things get to 65c before that. Some games don't make the system more noisy than idle, others do and of course stress testing does. Vysnc helps keep temps down as well.
Noise is subjective, but I'm a freak about it with my system. Generally 120mm fans at 900rpm or less or 140mm fans at 800rpm or less are the pinnacle of a low/silent sound from my experience. On a few fans I will find myself adjusting them to be faster RPM's to alleviate what I hear as sound distortions when they spin at certain speeds.
I've got a HTPC case in a very cramped shelving area so it's a unique challenge to get a high performance but still have a transparent sounding HTPC. I've got my PSU with 1x120mm fan, 2x80mm exhaust fans, 1x140mm intake, 1x120mm exhaust from it's rack position in my tv stand, and of course the video card and CPU HSF. Both the CPU and GPU are silent until load, fortunately they generally only load when I've got blanketing sound from the HT coming out. But for the quiet scenes in movies and during favorite music tracks ya need the thing basically silent on low loads.
Takes a lot of tweaking if you're working with fans and a system that wasn't initially designed around silence. Silence or even a quiet system is something you have to plan for from the start rather than throwing stuff together over a system upgrade cycle like i've kinda done on my system. I have too many different brands of fans. I'd go with 3 or 4 TY140 fans from Thermalright if I did it today but i'd still likely be stuck with 2x80mm exhaust behind the CPU that have to be used.
My relative measure of silence is one of the 1-1.5" deep iMacs from 2ft sitting distance. You can hear the fan, but it's easily transparent or obscured by the lightest of environmental sounds. With that measure being a 10 for silence and giving the irritation of noise by being 2 ft from a clothes washer (or an old delta fan for those that have used them) a 1, id give my system an 8.