Another sign if your heater core is leaking most of the time the inside of windows will tend to fog up in the wintertime.

 

I hope you mean that they'll fog up when you're not in the vehicle or something along those lines.

They'll always fog up during the winter due to the fact that you're breathing when you are in it (unless, of course, its set on defrost and has had time to warm up).

Sorry if I misunderstood what you said.

 

the windows foging up up is in reference to the fact that if the core is leaking there will be a supply of liquid inside the vehicle even when there is nobody in the truck. so if you are constantly coming out in the morning, or after your truck has sat parked for a while, and there is condinsation on the inside of the windows you either have a leaky core or a something else causing moisture to get into the truck... like a leaky weather strip or just a wet pair of jeans balled up behind the back seat from when ya went swimin out at the lake over the w/e... yes that one happened to me. I replaced the core before I realized I was missing my fav. pair of jeans.

 

the sloshing is lack of fluid. so either air or pin holes in the heater core could be the problem. If you dont see a puddle or smell antifreeze tan most likely a top off of your coolant will do fine.