The heater core lines are above the rad line, so it shouldnt dump much coolant out.
You'll lose like a 2-3 quarts of coolant when you back flush the heater core though, so when you re-hook the lines back up, top off the radiator with coolant and you should be good to go.
I just assume the sloshing sounds happens on start up alot or at random.
And to find the AC drain, just run the AC and wait for water to come out. I dont recall the exact spot but it's where on the lower half of the firewall on the passenger side.
