There are several reasons why this can be occurring. The pump motor bearings are failing and the motor needs to be rebuilt or replaced. There could be a partially blocked suction line causing pump noise in the way of cavitation. The pump could also be oversized for the application causing the pump noise and possibly [...]
If the water loss is more than evaporation and splash out would allow for, than you may have a leak and further diagnosis is required.
Evaporation averages up to approximately a ¼” per day (1¾” -2” per week). This can vary depending on, landscaping, air and water temperature, humidity, and windy conditions among other things.