After you drain and fill your Grobo, the unit responds to the recipe you have selected and begins a series of nutrient dosing. You will hear 2 - 3 pumps come to life and dispense food into your reservoir for your plant.
But what about a pump coming on when you have not drained and filled your unit? Is something amiss?
The unit not only doses food, but maintains a balanced pH according to your recipe. The pump that you are hearing is automatically adjusting your pH if needed throughout the day.