Pipeline Plumbing

This is the top cap and can be found in a plumbers merchant, how ever if you cannot locate this you may need to replace the flush valve itself.

Yes silicone can help solve this issue

A plumber will isolate the water, then replace the compression joint making sure there is new olives fitted. also it is a good idea to use ptfe tape.

This maybe due to an airlock.

Whatever the mains feed arrangement to the communal tank, after any of the flats have drawn off water, the ball valve opens & allows mains water to fill the tank back up.
the noise is probably due the ball valve failing or limited flow into the communal tank and starts to subside when the ball valve starts closing off.
if the flow through the ball valve is less than normal, the final closing off stages, could be over an extended period.
the shower & taps problem could be related, if that water is being drawn off quicker than the ball valve is filling the tank up, and the tank has emptied causing air to enter the pipes. replacing the ball valve should solve this