Here is a link about the specs for PWM fans.. http://www.formfactors.org/developer%5C ... Public.pdf
and the specs page link for the DS14.. http://www.netapp.com/us/products/stora ... specs.html
(about 1/2 way down the page).
And also a link about NMB fan specs.. http://www.nmbtc.com/dc-fans/engineerin ... tions.html
. The page also has related links to "white papers" and other fan specs, such as sensors and speed control.
That PSU in your unit is rated at less than 50 dB sound level, normally.. so either the original fans have been replaced at some time, or they have impaired air flow which can raise noise levels.
Measuring the output from those fan sensors is not accurate unless using an oscilloscope or frequency counter. The output is a rapid DC pulse, roughly a square wave form. A DC measurement would only be an "average" level.
While some models of the NetApp line do have a BIOS feature, yours may not.. but would still rely on a ROM program for operation. And a very basic mainboard setup, such as the Arduino® micro boards.
Have not worked with network setups in years, but remember the hassles with working on the old HP Netserver Pro systems.. the "beasties" that howl! The only way to access the BIOS on them was with a special setup/service diskette.
F@H.. to solve mankind's maladies.. in our lifetimes!