On a 2004 silverado with a 5.3 I am very slowly losing antifreeze. No visible leak anywhere. What are the probable causes?

If you haven't found the problem already. I suggest looking at the drivers side radiator tank they usually crank and seep slowly. If not leaking there, it's probably the intake gasket. Both of these problem are common to this make and model.

drain the oil an see if it has water in it. if it does that you have a big problem. if not that make sure the freeze plugs are in other than that umm...idk if there is no leaks than it has to be draining into your oil

If you are sure it has no leaks and this has been verified with a pressure test being done by a shop then you may have a leaking engine cylinder head gasket. I would suggest that you get under the car and look for white powdery deposits that are left by coolant leaks, also have the system pressure tested, this requires a special tool.

SOUND LIKE HEAD GASKET LEAK.CHECK ENGINE OIL FOR SIGNS ANTIFREEZE CONTAMINATION..OIL WILL LOOK LIKE MILK SHAKE.IF USING WATER IT EVAPORATES AT 212 DEGREES. ENGINE COOLANT TEMPERATURE CAN CLIMB UP TO 220 DEGREES.IF OIL IN CRANK CASE LOOKS GOOD.ADD 50/50 WATER AND ANTIFREEZE. ANTIFREEZE WILL RAISE BOILING POINT TO KEEP COOLANT FROM EVAPORATING.SEE IF YOU LOSING WATER ITS BOILING AWAY LIKE WATER IN A TEA KETTLE.

It could be a slow leak in the radiator or hoses or the head gasket. The antifreeze could be blowing out of the cylinders rather than leaking to the ground.

thermastat problem? Antifreeze not circulating properly? Radiator needs flushing?

check under the water pump for stains from dried coolant, that is the common cause here.

intake manifold boltsare loose goto alldata.diy.com to confirm soulution.

It probably is a head gasket, unless you can smell anti-freeze inside the car, then that probably is very small leak in heater core.

how often does the warning comes on doing the week or in two weeks? if it was a one time thing or this takes place every 3-5 months i wouldn't worry about it. i will make sure that the radiator cap is on tight. you said that all hose seem to be in good condition i would check the radiator where the cooling fan covers the fins and the water pump because it has a warn hole at the base that will seep out antifreeze when the inside seals are going bad these will cause the smell also.