Most of the noise on 3d printers comes from the fan blades. Like laptops, too much heat can cause the extruder fans to work overtime to cool it down, hence the sound.   People know Prusa printers for the loud welcome. The Mk3s greets users turning it on by whirring at full speed for a few seconds before quieting down.   Per Prusa’s customer support, part of the start-up process involves clocking the fans to 100% RPM. This ensures that the fans run after its Power On Self Test (POST).   This step ensures no overheating happens because of a bad fan. There’s nothing to worry about, and no need to replace the fan or any other part in this case.