Engine temperature, too cool

While driving, temperature gauge drops from normal center temperature to coldest (looks like it just goes limp). When this happens the air won’t blow cold. If I pull over and turn car off and back on a few times it resets to normal, mid range temperature on the engine temperature gauge. When I turn the car off (only after gauge has dropped) I hear fans turning (at least sounds like fans) for a few seconds, this doesn’t happen if car is turned off during normal temperature reading. I’ve had a couple times where I turned the car back on and it made a terrible grinding noise and seemed like grinding and idling at the same time. Gas mileage is terrible. Noticed low coolant in the reserve in car tonight so I added coolant but that didn’t change anything. Just bought car eight months ago.