I can hear fluid circulating

Check your coolant level. If it gets low, air will find it's way to your heater core, and will slosh around.

Does it seem to be running any warmer than usual?

I took it in and the garage did a coolant pressure test. They said the pressure was low and it was leaking fluid from the cap. They replaced the cap. I can still hear the noise, but not as bad. The temp is fine, it stays around 1/4 on the gauge, and only rises a bit when idle, but never above half way. And it doesn't spike either, which might indicate air in the coolant. Is there anything I can do myself to try and get the air out if there is any in there ?

The only time I heard coolant flowing was when the antifreeze had frozen up a little bit and the ice chunks were flowing through the cooling system. What is your amtifreeze to water ratio?? If your in Florida then I don't know what else to suggest.

========= Harryface =========

1991 Pontiac Bonneville LE, 3800 V6 _~_~_~_~273,777 miles_~_~_ ~_~_

I'm in Canada and I use the premixed store bought stuff. It is good to like

40 below or something, so I doubt it is freezing. Besides I can hear it even when the engine is warm. If I am standing over the engine, and I rev it, I can't hear it, perhaps cuz the engine is loud from there, but I can hear it when I am sitting in the car.

Air in the heater core isn't that uncommon. (Hey, don't they inject compressed air in to the system to do a pressure-test?)

The heater core is a radiator on it's side (usually). It takes a while for the air to work it's way out of there once it gets in there. Air in the heater core won't reduce coolant flow in the engine. Once the air gets out of the core, it bubbles it's way to the radiator (or the bleeder valve on the thermostat if it has one).

This sloshing sound is when I know I need to add more coolant in my '94 grand marquise. After I add coolant, it still sloshes around for a while.

These engines have a problem of intake gaskets gradually leaking coolant into the engine. Check the bottom of the oil filler cap,dipstick. etc. for a milky white goo,and keep you`re eye on the coolant level. As the level gets lower air will enter the heater core.