I wonder that also, recently accidentally ordered a 5V one for my XeonD system. It works fine, but my expectation that it would be less noisy is not true - or its not pwm controlled after all, at least it never turns down. Have not delved deeper since the board is still on the bench and real life catched up...I believe I'll probably pick up a Noctua 40mm fan, question is what's the preference/difference between these two or is there a better option?
Amazon.com: noctua nf-a4x20 pwm
One is a 5V version, other looks std, both PWN 4 pin, not sure the benefits or if either will work.