you have nothing to worry .....
Fanless Mode means just that; the fan blades are completely stationary. There is no audible PSU noise in Fanless Mode, which is active when PSU output is below 20% of the rated DC output load. This completely silent mode is feasible due to the extremely high efficiency of these PSUs.
Quiet Mode runs the cooling fan at very low RPM, which results in extremely low level audible noise. The noise level is <16dBA, and is active when PSU output is between 20% and 50% of the rated DC output load.
In Cooling Mode, the fan speed and the associated airflow is adjusted to optimize thermal performance. This mode is based upon DC loading and ambient temperature conditions, resulting in the most effective and accurate cooling performance under heavier loading. Cooling Mode is active when the PSU output is above 50% of the rated DC output load.