2000 K2500 454, auto, a/c, 173k +

Lately I have been hearing a water noise in the dash area when turning or moving. Is my heater core leaking and filling up my heater box? I have had to add antifreeze to the overfill jug, but I don't see any external leaks. Sometimes I do get a whiff of coolant when I turn the heater on. If so, is it a big job to repair? I am hoping it is something else but, I don't have any other ideas. Is there something I should look for? Should I pressure test the cooling system?