The noise as Paul points out is normal. The lack of noise is the abnormal condition.

I think this was pointed out earlier, but if not it is a tiny compressor in the unit that purges the sensor tube with air. Then the pressure from the column of water in the holding tanks is measured and used to light the level indicator lights.

If the sound is annoying turn off your water and holding tank sensor system.

Think of it like you do the engine. The engine makes noise when it is working.