No Coolent Pressure

I have a 99 Silverado with the 5.3L engine and 56000 mi. The problem I have is the coolent system does not build pressure. There is also a gurgling noise from the heater core when the engine reaches operating temperature. There does not seem to be any loss of antifreze since the coolent level in the expansion tank stays around the full cold mark.

I have replaced the pressure cap, but the problem remains.

Other than not building pressure and the gurgle, the truck drives fine, no coolent in the oil and no coolent smoke from the tail pipe.

Any ideas on the problem?

Thanks