How much noise does the fan on model D2 Clone/D2-310/D4-310/D4-300/D5-300/D5-300C/F4-300 make?

Model D2 Clone/D2-310/D4-310/D4-300/D5-300/D5-300C/F4-300 is designed to be quiet. In addition to the noise from hard drives, he fan registers only 22 dB, which is appreciable only in very quiet environments.