I replaced mine with a set of
ARCTIC S4028-6K fans from Amazon for $40 for a set of 5
These are PWM fans with a 250-6000 RPM range, they also make a 1400-15000 RPM variant
They're much quieter than the
stock 4028B12M fans for my use case (a home router for 10Gb WAN so not really pushing it hard at all) and are the full 28mm length to reach the shroud, since it's a pack of 5 there's also one extra to fill the hole between the power supplies!
They are 7.1 CFM @ 6000 RPM compared to the stock 12.95 CPM @ 7000 RPM
Something of note, I'm not sure that the device is taking full advantage of the PWM functionality stated here:
https://help.mikrotik.com/docs/display/ROS/Health
For devices with PWM fans, as the CPU or SFP temperatures exceed 58C, the fans will linearly increase their RPM to try to keep the temperature at 58C if possible.
However that doesn't seem to be happening for me, I'm still occasionally seeing a cyclic on/off behaviour where the minimum fan speed is around 2500 RPM (this was after I ran fan-detect) It's also jumping from ~2500 RPM to ~5000RPM with no ramp up, as if it's still using voltage jumps to control the RPM. I'm not sure if the quoted statement applies to the 7.9 release like the section before it so I'm going to see what happens with 7.9 when it releases
Additionally I haven't figured out a way to make the 5th fan appear in the System Health panel, but it seems to behave the same way as the others.