I was always unhappy with the noise my Kessil's generated when turned on. I RMA'd one and it's replacement was a little quieter, but after a few years it finally got to me and I did the fan swap.
It is now inaudible. I used Noctua NF-A4x20 and it fit perfectly. Here's the parts I used
If you want to directly exchange the stock fans these are what I used: Noctua 40mm 12v fan for the motherboard and exhaust fans. You’ll also need this step down converter because they are 12v fans instead of 24v but they work well and are nice and quiet. I also keep the exhaust fan turned off all the time unless printing with ABS. Even with a noctua fan replacement in there it’s just so loud because its in a huge echo chamber. For the extruder fan I used this 30mm 12v fan and again used the step down board to reduce the voltage to 12v. There’s plenty of extra room in the extruder head to fit it in there and I haven’t had any cooling problems since installing it.
I'm using the Noctua 40x20 FLX on my Ender 5 and Ender 6. The former has a Micro Swiss all metal hotend (non-direct drive - just the stock mk8 hotend replacement), the latter a Micro Swiss NG.