The gurgling sound is air bubles rushing through the heater core, and yes, the fill needs to be completed. With the engine cold, check the overflow bottle. The liquid will be its minimum level and should be above the "low" mark on the bottle. If not, add coolant until the bottle is half full or at to sligtly above the cold level mark.

Here's whats supposed to happen. When the coolant heats up, it expands and should burp some some trapped air into the bottle. Then, when the system cools and the coolant shrinks, it draws fluid from the bottle into the cooling system, that is, if there is enough fluid in the bottle to keep the inlet/outlet in the bottom of the bottle covered.

After about 3 thermal cycles, the system should have enough air flushed out. After that, keep an eye on the coolant level in the bottle for the next 3 to 4 trips.

The "sealed" coolant systems on today's cars vary in detail such that no one set of descriptions fit all. However, all sealed systems perform the functions described above. The mechanical details do vary. In some systems, the overflow tank is pressurized to the same level as the engine coolant, while in others, the overflow tank is vented to the atmoshpere.

This is what I'd do. With a cold engine, fill the overflow bottle to or slightly above its "cold" mark. Put the cap back on the overflow bottle. Warm the engine into its operating temperature range, then shut off and let cool. After cooling, refill the bottle. Do this until the liquid high and low level stablizes.

Be very careful! Do not remove the cap of a pressurized bottle if the coolant (engine) is hot. Upon release of pressure, the coolant can flash to steam and geyser about half of the coolant out of the system. You could get scalded, not to mention the ensuing mess. Let it cool until you see the coolant being sucked out of the bottle back into the engine.

Antifreeze can be purchased as full strength, or already diluted half water and half antifreeze. Look carefully at your antifreeze container. Do not dilute the already diluted antifreeze.

jem is correct about air bubbles trapped in the heater core making the "gurgling" sound. Most of the time we have to open the heater core hoses and bleed the air out of some cars manually. That is what the shop will have to do in order to get rid of the noise.

For the most part it wont hurt anything, except possibly reduce the heating capacity inside the car, and sound annoying.

Thanks so much to both of you for responding. I'm going to check the overflow bottle's level before I leave this morning. Thanks again!

I've hit that annoying gurgle and turned on the heater so the valve lets the coolant flow freely thru the heater core to get it filled bubble free after a couple of engine hot/cold cycles.