One of the biggest annoyances of this printer is that the UV LED array cooling fan runs 100% of the time. Even when there isn't anything being printed on. 

It wouldn't bother me so much if the fan wasn't so loud. Which at full speed, distracts from the printer's elegant operation. 

Thankfully there is an easy fix. Using the printer's configuration file, we can tell the printer to turn on the fan only when it is actively printing. But, if you're like me and still feel the noise is too much, lets mod the printer to use a whisper quiet Noctua Fan.

Just shush if you are slacking...

To run the fan only when printing, you need to find the printer's configuration file in the root of the USB drive that came included with it. If you have deleted the file, or have lost the drive, no worries you can download it directly from Elegoo's website. It should be under Mars 3 > USB Drive Files > English Language

The file in question is called mars3ChiTu Mainboard SpecificationV3.0.920210812.gcode.txt. Despite being files in "English" you'll find the following lines within... For me it was close to lines 95-104:

;【风扇1控制】    
M8030    I-1    ;>0: 如果需要强制风扇1大于某温度时自动开启风扇，设置一个大于0的温度.
            ;0: 如果设为0,默认不转
            ;-1:一直开            MF
            ;-2:文件打印时开，没打印时关闭    EF
;【风扇2控制】    
M8030    T-1    ;0: 如果设为0,默认不转
            ;1: >0一旦曝光，风扇2就转，不曝光，风扇就停止  LED
            ;-1:<0一直开            MF
            ;-2:文件打印时开，没打印时关闭   EF

Using google translate we can decipher a little bit of what this does:

;【Fan 1 Control】
M8030  I-1        ;>0: If you need to force fan 1 to automatically turn on the fan when the temperature is greater than a certain temperature, set a temperature greater than 0.
                  ;0: If set to 0, the default does not turn
                  ;-1: Always open MF
                  ;-2: open when the file is printing, close EF when not printing
;【Fan 2 Control】
M8030  T-1        ;0: If set to 0, it will not turn by default
                  ;1: >0 Once exposed, fan 2 turns on, no exposure, fan stops LED
                  ;-1:<0 always on MF
                  ;-2: open when the file is printing, close EF when not printing

Here there's talks about two fans, however there is only one fan in the printer. There is an unpopulated header in the Motherboard named LED, that might be related to this, maybe well come back to that one later.

I ended up switching both lines to -2 for both instances:

;【风扇1控制】    
M8030    I-2    
;【风扇2控制】    
M8030    T-2    

This will turn on the fan only when the printer is printing.

Save the file to your USB Drive, plug it in the printer, and browse for the .txt file and print it as if it was a 3D object. It will set the printer settings and it should stop the fans from going all the time.

Whisper quiet operation...

Now let's work on replacing the fan. The process here is straight forward, I'm not going to go into much detail as it has been covered in the past., after all is just replacing one fan for another. There is only one caveat to this mod, the fan header and the fan this printer uses are 24V, and, most often than not, the Noctua Fan is 12V.  Therefore, we need a small DC-DC converter to lower the voltage, or if you desire you can also use a voltage divider using a couple resistors.

During this process I decided to also use the excellent air duct from Thomas Linke. I will attach some of the steps I followed for this process. While he used TPU filament, I decided to print everything in PLA.  According to him it doesn't get too hot in there, we'll see...  

I wanted this duct to be semi-permanent. I decided to glue it in place in most places. assembling the unit outside is much easier. but it requires complete disassembly of the printer. (Which is easier than I anticipated this printer is so simple!) 

Conclusion

After this air duck, fan mod and settings combo, the printer is whisper quiet during operations. You can't even tell it's on! if it wasn't for the motor humming along, you'll think it stopped.  If you want to make it even quieter, you can adjust the DC-DC converter to output less than 12v, about 7.5v is usually a good value for slower speeds and noise.

Highly recommended mod! 

Let me know in the comments if you have any questions!