I have built a 3D printer from parts. It is using a standard 12V power supply, an Arduino Mega 2560 replica and a RAMPS 1.4 board. The hotend cooling fan is connected to the 12V-AUX pin (the one right next to the x axis stepper driver) on the RAMPS board so that it continuously receives power as long as the machine is turned on. The printer is controlled by the Marlin firmware.
When I give power to the board, the cooling fan starts spinning with a lot of noise. It sounds like it is receiving a higher amount of current than it should. Moreover, the noise is not steady, but fluctuates slightly. After about 30 seconds, it gets slightly quieter; after about 40-50 seconds, there is a sudden break. From then on, the fan spins quietly and regularly.
According to measurements with a multimeter, the power supply is keeping a constant and correct voltage of slightly over 12V right from the beginning. Hence, I suspect the issue is somewhere on the RAMPS board.
I am worried about this being more than a problem of unpleasant noisiness. What could be the cause and is there more to worry about?
You can listen to a recording of the sound: 3D printer hotend cooling fan noise
Note that I am not talking about a fan for cooling the extruded plastic, but the one sitting on the extruder heatsink.
Shutting down the printer (even literally unplugging the power supply) and plugging it back in does not lead to the startup noise appearing again. Only when waiting for multiple minutes before reconnecting does it happen again.
The voltage to the fan is measured to be constant. The current, on the other hand, correlates with the sounds heard. It starts at 110 - 115 mA. When the fan is quiet, it is at about 90 mA.