The heater bubbles despite the automatic quick vent

Our heating system gurgles very loudly several times a day, although we have an automatic quick air vent at the highest point on the upper floor (whose black cap is loosely turned).

Does anyone have an idea what the problem could be and how to fix it?

1) You can't manually vent the quick air vent, can you?

2) Is the problem that there is air in the heating system that does not escape through the quick air vent (because it is somehow defective)? Or is the problem that the quick air vent works but draws in air in certain situations? Then I would have to turn off the heating system and after some time close the black cap before I turn the heating system back on.

Depending on how your heating is installed, the air is not pushed all the way to the fan. The air stays in place and the water gurgles past. Can you locate the noise?

Can you locate the noise?

I hear the gurgling most strongly in the two distribution boxes on the ground floor and upper floor at the upper of the two horizontal pipes. The two automatic quick vents are also on these horizontal pipes. However, the gurgling does not occur continuously, but only several times a day. I tried closing the two black caps. However, that had no effect. Could the two quick vents be clogged so that the air can no longer escape? Could it help to replace the automatic vents with manual vents?

is the heating in the basement and how many floors are there above it up to the air vent, 10m height corresponds to 1bar. if the pressure in our old house fell below 1.5bar (at about 12m height), the heating also started to gurgle

Is the heating system in the basement and how many floors are there above up to the air vent, 10m height corresponds to 1 bar. When the pressure in our old house fell below 1.5 bar (at about 12m height), the heating also started to gurgle

It is a single-family house (basement + ground floor + upper floor) with the heating system in the basement.

The upper floor floor level is 5.60 m above the basement floor level. Pressure is 1.8 bar.

If the two distributors have no air vent, like radiators?