I have it running all day, everyday in my studio.
I've never heard it.

I have two M7. One bought new and is very quiet and the other I bought used, and is super loud. Maybe that's the reason why the guy sold it to me for a good deal.

I bought a Noctua 12V, 40mm x 40mm x 10 fan on Amazon for $13, replaced the noisy fan and it becomes even quieter than the new one. It is super easy to do. All you need is a proper size screw driver.