Car runs hot while idling, then cools down when you start driving. Have to add antifreeze but cant tell where it is leaking. could it be a heater valve busted or is it possibly the water pump?

It`s easy to see if the water pump is working just drain some of the fluid out of the radiator and look inside while vehicle is running NOTE due this only when vehicle is cold.
Know if it`s not flowing after it starts to warm up then either the thermostat is stuck closed or the water pump is bad i would start with the thermostat if i where you the other job is a lot harder and more expensive
Also the fluid disappearance could be caused by the fluid boiling out when it gets hot one last note check pressure cap on radiator for wear or cracks if bad replace hope this helped you.

If there is no visible leak (dripping or puddle under car after parking), check the engine oil. If it's milky white, your coolant is leaking into the oil (head gasket is bad). When the car runs hot when idling, is the cooling fan kicking on? If not, the switch that is supposed to signal the fan to turn on at a certain temp is defective, or possibly the fan motor is bad. Check the water pump for signs of leakage. There is a weep hole at the center under the pulley that spins the pump. Use a small mirror to look for evidence of a leak there. If you find it, replace the pump, as the leak will get worse, and is a sign of the bearings going bad.