It was said by several and Ill add my own...
If it sounds like a brief 2-3 seconds of something "groaning" with a sorta rubbing/sucking sound, yup Recirc Door groan...
Gurgling is always Antifreeze vs Air in the heater core/cooling system in any vehicle... either a system leak or air pocket.
All SBCs can have air pockets, regardless of what they are in. The heater core in these trucks can definately form an air bubble pocket that doesnt simply flow out, because the heater core is partially above the top of the radiator. Easiest way to clear it is to park it on a hill or put it on jackstands, so that the nose/radiator is higher up than the dash and bleed it.
- Run it up to operating temp on the highway, then Park it on a suitable hillside/jackstands and jam the Ebrake pedal nice and tight. You need the top of the radiator to be higher than the heater core. Shut it down and let it cool off for at least an hour, allowing everything to cool, contract, & depressurize.
Before you start, make sure the radiator is warm, not hot to the touch, if otherwise, give it more time to cool off. Open the radiator cap SLOWLY, to fully depressurize the system. Be absolutely sure you open it very slowly... you dont want to have an antifreeze volcano and get 3rd degree burns... be prepared to close it fast.
Completely open the rad cap, but leave it sitting on the radiator's cap hole kinda ****-eyed to prevent AF from splashing all over. Fire up the truck and let it run for about 30 seconds, then shut it off. Top off the antifreeze in the radiator and run it for another 30 seconds... top off and repeat until you cant put anymore AF into the radiator. Lock the radiator cap down and take the truck for a good run... repeat if necessary... but it should clear itself of any remaining air. Also top off the AF overflow tank to the proper height because if its empty, it will suck air into the cooling system .