You have air in the system that has to be bled out
The gurgling sound is caused by pressure in the engine coolant system(antifreeze) slowly releasing into the coolant overflow resevoir. Typically a faulty thermostat causes this sound.
Your coolant is low.
That means there is air in the lines and must be bled out.
Low coolant and air in the heater core.
If there is air you will hear bubbling as the water flows. If you do not hear anything while the pump is on you are probably ok.
I am renewing my heater system because it is old. Where can I get more information on electric baseboard heaters?
A cooling system is supposed to be drained not just to rid the gurgling sound. It also helps the plumbers to clean and unclog any accumulated residue.
Air in the cooling system - has to be bled to remove air
Low coolant level, air in the cooling system. Low coolant level or a faulty rad cap allowing air to be sucked into the system when it should be sucking in coolant from the overflow bottle.
Probably due to an airlock in the cold water system (feed pipe).
About 150 deg.
A bleeder valve to remove air in the system