I updated the fans in here yesterday, I got some of the Noctua NF-S12B REDUX 120mm for the back and side, and a Noctua NF-P14s REDUX 140mm for the front.
Having used them in my File Server build I knew they were good, they have a much smoother sound than most, I have each set at 1000rpm in Thermal Radar so I hardly notice them, I do however notice the bloody Corsair H100i fans, which are next on my upgrade list (should have done it before lol).