Noises when drawing cold water

Then take citric acid... I just don't find the effect that impressive compared to vinegar

Descaling with vinegar is done when it comes to hot water. Cold descaling should be done with citric acid. You can also leave it in the system overnight and drain and flush everything out in the morning.

Once again for everyone: I suspect the problem area is in a pipe section below the basement ceiling. So higher than the tap points in the basement, and lower than the tap points on the ground floor. I have a drain screw directly at the main valve, and I can also empty the boiler at the base of the heating system.

Basically, I am only concerned with the cold side, because the noise only occurs there. But it doesn't hurt to descale the coil in the boiler while I'm draining everything anyway. However, I don't know how much water goes in there. Everything communicates with each other, which is why recently I hardly got anything into the system, because on the upper floor the connection to the shower communicates with the ventilation valve located at the highest point. Therefore, I would ignore this part of the system this time.

There must be about 200 liters in the tank, right?

Unfortunately, the inlet to the boiler on the cold side cannot be shut off. Because of this, I can never be sure how much water is actually in the pipes, and it may prevent the citric acid water from flowing into the relevant area from above.

Completion report:

The backwash filter had to be completely replaced because it was dripping and after such a long time no parts were available anymore. Apparently the pressure reducer in the old backwash filter was defective, probably also clogged by limescale, and it caused the noise. Since the new backwash filter was installed, the noise is gone.

Interesting though, and typical for resonance: acoustically and by feel, I would have located the problem source somewhere completely different.

But my actions with the citric acid were not in vain, because only after that did the dripping begin, and without the dripping I would not have had the backwash filter replaced.

It should only be backwashed every six months. Is that sufficient or should it be done more often?

Matthias thanks everyone involved!