Noises when drawing cold water

Of course, I have to have someone replace the backwash filter. But the presumably necessary descaling I have to do myself. How long is someone supposed to spend on that? Just the emptying alone takes forever, and then the stuff has to soak for several hours. The successor to my deceased heating engineer certainly didn’t seem to have any idea what I was talking about...

How long is someone supposed to spend on that?

Probably less time than you, since this person has the necessary experience and equipment.

Just the draining alone takes forever, and then the stuff has to act for several hours.

I am aware that pipe flushes usually take place within closed circuits. And yes, that can indeed take a few hours. Whether someone always has to stand nearby, I cannot judge.

The successor of my deceased heating technician certainly did not give the impression that he has any idea what I am talking about...

Then he is probably not the right contact person.

Probably citric acid.

Citric acid is only conditionally suitable for descaling, as it can leave behind citrate (a salt). Amidosulfonic acid is better and also gentler on metal. As far as I know, it can also produce "amidosulfonates," but not as strongly as citric acid, especially when heated. But it's better not to make it yourself. It's best to have a professional plumbing company do it.

Vinegar in large quantities 1:1 into the circulation over the cold water pipe might maybe do something... but you need a lot if you want to flush the hot water pipe (everything goes through the boiler anyway)

I don’t understand flushing over the faucet? There is pressure from the water pipe and even if you turn off the faucet maybe 2 liters are missing in the pipe and you can't really push back the air... does it go in this direction?
I think to make it work you would also need a valve in the basement where you can drain the water from the pipe to actually "flush" it

Definitely replace the backwash filter, but flush before that and then put in the new filter

I have now read and understood your idea

Then open the valve in the basement, let all the water out
Put vinegar in the fixtures upstairs 1:1 until it runs out downstairs, then close downstairs (put the valve on) and wait... after 30 minutes flush everything again with clear water

It should get better then... it would be bad if the dissolved lime has now caused a pipe to leak somewhere... then you do leak detection...

With vinegar, out of ignorance, I once wrecked the seals of a fixture…