ᐅ Air in cold water pipe: Loud noises despite normal water pressure?

Hello everyone,

For several weeks now, we have been experiencing air in the cold water pipe, which causes loud noises when water is running (for example, when the flush tank fills, the washing machine draws water, or when washing hands). You can also notice the water turning briefly cloudy at the faucet, indicating air in the system.
We live on the upper floors (1st and 2nd floor) of a two-family house built in 2004.
When I stand in the basement near the supply line or close to the expansion tank (refix DD 12) while water is running, I can clearly hear the noises.
Since I could not find any leaks or dripping pipes and the pressure seems fine (around 5.5 bar), I wanted to ask here if you can suggest possible causes?
I just pressed the valve on the expansion tank again, and only air came out...
I have attached a few photos. Could it be that air is entering the system through a defective component in the pipe run?
I hope enough is visible despite the poor lighting conditions.

Probably air in the pipe. If the pressure stays at 5.5 bar (80 psi), a leak is unlikely. The issue could be with the expansion tank or the venting. A damaged membrane in the tank might be causing air to accumulate, which creates the noises.

It would be good to bleed the expansion tank if you haven’t done so already…

To bleed the system, I first fully close the valve directly above, then bleed at the small green valve to the left, similar to how you would with a radiator.

Do I also need to close anything in the pipe going down to the hot water tank, or should I leave it open?

Yes, exactly, venting works like with a radiator. Turn off the large valve and open the small green one to vent. You don’t need to close the hot water storage tank separately – that’s not necessary as long as the pipe itself is not affected. Make sure the pressure in the tank is properly adjusted. If venting doesn’t make a difference, the tank itself might be defective or simply too old.

Hm, okay, I can only check the pressure in the tank using a suitable pressure gauge, right?
Then I would get one first.

When bleeding the system, proceed slowly. Check the pre-pressure in the tank (about 1.5 bar (22 psi)) and inspect the connections for small leaks. If air continues to come out, the diaphragm may be porous.