I have a 1991 Chevy 1500 with a 5.7 engine. I have to add coolant to the radiator regularly and I don't have a leak, inside or outside. I have a liquid sloshing noise in the dash along with a stale water smell through my vents. What could this be? No fogging of windshield. No wet floorboard. Plenty of heat when needed. Need to correct before it gets worse. Where does anti-freeze go when there's no evidence of a leak?
This could be a sign of a head gasket problem. If you're loosing antifreeze and have no evidence of it on the ground and in the truck then it may be being burned in the engine. The sloshing sound you hear is air in the cooling system circulating through the heater core.
You may need to pressurize the cooling system, take out all of the spark plugs and wait a few minutes. After a few minutes; crank the engine over with the spark plugs out. If there is a head gasket leak you will see antifreeze blow out of the spark plug hole(s) where the leak may be.
Try this out and get back to me when you can with the results. We can go on from there.
I was afraid you might say that. Will check on Monday and let you know. Thanks for your help.