. . .When the water level in the sump drops (evaporation) it will also lower the water level in the overflows. That creates some noise from the water falling.
My overflows don't drop unless the pump starts sucking air. Then I will hear it and add water. This never happens if I keep the normal WC schedule, unless the sump drops because the overflow strainer is plugging up. I can hear the pump gurgle and I know to check it.
BTW, I do NOT run totally silent systems. I like the noise, and also if the power goes out at night I will wake up from the sudden silence. A cavitating pump will wake me as well.