Im using Noiseblocker NB-Multiframe M12-S1 120mmx25mm Ultra Silent Fan (750 - 2000 RPM)
replacing the exhaust fan on Acer AH342 Atom 510 4hdd server, the fan is really good, but the server has it running at 1350rpm where is not quiet, but i needed the extra cooling, but below 800rpm is really good, depending on how your mobo can control it. There is a lower rpm version that might be worth looking, Noiseblocker NB-Multiframe M12-PS 120mmx25mm Super Ultra Silent PWM Fan (600-1500 RPM)
, worth mentioning that the fan structure while fine for mounting on cases, idk how it will work on a CPU cooler, will depend on the mounting mechanism, noiseblocker offers also the more tradional frame fans in case you need it for the mounting, Noiseblocker NB-BlackSilentPro PLPS 120mm x 25mm Ultra Quiet PWM Fan (600-1500 RPM)
Scythe 120mm PWM fans have a much wider range of rpms,Scythe Kama Flex 120mm x 25mm PWM Fan w/ S-Fluid Dynamic Bearings (SA1225FDB12H-P)
- I own one, spinning at 300rpm controled via bios, on my build
, the blades have very similar design to the sflex.Scythe Slip Stream 120mm x 25mm PWM Mid Speed Fan - (SY1225SL12LM-P) (200-1300rpm)
- I think this is the same that comes with Mugen 2/3, reviewed on SPCR Scythe Mugen-2 CPU CoolerScythe Slip Stream 120mm x 25mm PWM High Speed Fan - (SY1225SL12H-P) (300-1600rpm)
My favorite PWM fan atm is Thermalright TY-140
, according to specs it 900R~1300PM, but in my case its running at 700rpm (not sure if its right or not recognized correctly), but at 700rpm is very quiet and moves a lot of air, something interesting about the fan is that as my Scythe Gentle Typhoons (i run 5 in my antec 1200), this fan is the last fans to spin down on power off, at least 5 secs more than my Noctuas and Scythe Sflex/Kamaflex, but i dont think you can mount it on the Coolermaster V10.
What im really looking foward is Scythe releasing a Gentle Typhoon PWM, but who knows when or if it will ever.
Recaping, i would probably go with Scythe Slip Stream 120mm x 25mm PWM Mid Speed Fan - (SY1225SL12LM-P) specially for the price and the fan has wide range, from 200rpm (virtually inaudible) to 1300rpm (where it might be loud but still a good number in case you need it when OCing). But if you want to test something else beside Scythe i would go with the Noiseblocker NB-BlackSilentPro PLPS 120mm x 25mm Ultra Quiet PWM Fan (normal frame to avoid issues with the mounting).
Sooner or later the RPM is going spin up at some point. You'll end up dealing with the loud fans when you push the system personally it's why I avoid PWM and stick with stardard low noise fans.
This depends a lot on the mobo, in my case, the intel DH61DL has the cpu fan at 700rpm and even under prime95 it wont ramp it up, i think its because it doesnt reach a thermal point where it will ramp it up, but there are some mobos that you can setup the ramp ups and how much you want it to go up, check ASUS Sabertooth 990FX Motherboard Overview (AMD Bulldozer) | JJ's Corner
. In my case i like a lot PWM fans, specially on high performance rigs, where in some scenarios you might need the extra performance with OCing and its great to have fans that will react to the conditions of the pc, as long as you can set it up to your personal preferences.