Loud. Very noticeable. Fortunately there's an extremely easy mod to quiet these fans . All you need to do is splice a zener diode inline on the positive fan lead. Do a search for "Fidek fix" on this forum to get the part # and instructions. Takes about 5 minutes to do.
I do believe this is the Zener diode needed for the fix: $1.50 parts express part
On a side note, there is a website somewhere that shows this project being done (maybe by Mike Knapp?? or maybe the link is on his website??) complete with pictures and instructions. I am going to continue to try and find it, but if someone has the link handy, please post it.
Kevin