I think we all have different ears... and certainly different background noise environments
What seems quiet to me during the day with traffic and other back ground noise seems very different at 1am...
My suggestion to approximate one PC noise to another is to multiply the rpm of a fan by it's blade diameter.
If one of these is significantly more than the others then it's the noise level for the PC, if they're all similar then add few % for each additional to the highest one. Obviously this disregards the hard drive and any restrictive vents and poor fans/bearings etc but to get some idea of one PC to another.
I find in quiet environment that anything over 90,000~100,000 "rpm mm" starts to be audible. (ie 120mm fan with 110mm blades @1000rpm = 110*1000 = 110,000 rpm mm)
My PC has [under CPU + GPU Folding@Home]
CPU fans: TY-140 x2 @ 800rpm = 104,000
Rear case fans: F9 x2 @ 1000rpm = 84,000
Lower case fan: F8 @ 1150rpm = 82,800
GPU fans: F9 x2 @ 700rpm = 58,800
PSU Fan: F8 PWM @ 900rpm = 64,800
We can ignore the drives as SSD OS drive and storage drive is WD Green in a Scythe Quiet Drive sitting on soft foam.
CPU fans +10% for 2 of and -20% for being inside the case = 91,500
Rear case fan + 10% for 2 of + 10% for lower one = 100,800
GPU fans +10% for 2 of and -20% for inside case = 51,700
PSU Fan = 64,800
This all seems right to me, the GPU fans don't seem to make any difference idle to load, neither is the PSU noticeable. Also increasing the case fans increases the noise more noticeably than the CPU fans, unfortunately with the amount of heat inside they have to work hard...
From the above figures I'd ignore the GPU and PSU and as the CPU fans are close in noise to the case fans add something for them to give my PC a rating of 110,000 "rpm mm"
So I'm saying my whole PC is about as noisy as a single 120mm fan at 1000rpm, hmmm will have to find one and compare
Idle is all fans 600~700rpm for ~80,000 and virtually inaudible, even at night.