Common problem for many bands of cars. MKIII Toyota Supras are very bad about it (don't ask).

My car was so bad, the factory had a campaign to install a 'fix it kit'.

Air gets trapped in the highest place, and can't get out. This is one good reason we (at my shop) use a machine to flush, and install coolant.

A pretty good home remedy you can try, and has worked for me many times.

With a completely warm and running engine, raise the front of the car up until the muffler tips are close to hitting the ground. In other words, as high as you can. I use a drive on hoist at work, or those ramps from autoparts store. A very steep hill will do the same.

Get the radiator top way up.

Now, with the heater on high, rev the engine to 4k rpm for about 10 sec. Let off for 10 sec. Repeat 10-20x. This will help allow the air to find its way to the top of the radiator, and eventually on to the recovery tank.

It is very bad to allow the 'gurgling' to stay for long periods. The air/coolant agitation WILL eat away at the heater core and cause a leak. Leak=$$$$$$$$$$$$$$$$$$$$$$$$$$.

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1990 300CE. Garage kept all its life. Looks & Runs Like New. Everything works. White w/Palomino int. Pioneer 12 CD player in the boot. 17" Enkei LX-F Chrome wheels w/Falken Azenis ST-115 225-45 tires. Tinted windows. Black sports grill w/Euro clear front lamps.