I would guess the cooling system has air in it = air-locked. Today's vehicle cooling systems are closed, no leaks or spills onto the ground. Most have a high-point air bleed device, whether it be a bolt to be loosened, or other means, on the engine. Often all it takes is several heating and cooling cycles to purge the air from the system if there isn't much air in the system. Be very careful not to overheat the engine if you choose to operate the engine trying to evacuate the air that way though, hot spots can ruin your engine if coolant is trapped by an air lock.
2000 Winnebago Ultimate Freedom USQ40JD,
ISC 8.3 Cummins 350, Spartan MM Chassis. USA IN 1SG 11B5MX,Infantry retired;Good Sam Life member,FMCA." My fellow Americans, ask not what your country can do for you, ask what you can do for your country. John F. Kennedy