I run my s9 as a heater during the winter and also do the solo mining, but don't get the noctua fans because they are not compatible with antminer (at least not the ones I bought).
Discussion
Would you r comend a quieter fan for an S9?
No, the only I bought a muffler. I have seen other solutions that use a box to hide the sound of rubber pads to go in between the fan and the unit.
I used the fans they mention in this document and run it next to my desk all day.
https://www.cryptocloaks.com/the-future-of-space-heaters-s9-bitcoin-asic/