Seem to have inadvertently "fixed" a banging drain issue but no idea how it makes sense

[ElCI]

Hi all

Moved into new house a few weeks back. Since day 1 we've had an issue where when running any water in en suite we get this clicking/banging noise at high level but I feel like it's coming from the drain (pipe chase is in en suite).

Separate to this there's been a weird thing where there's a pipe outlet for my drain that seemingly connects into the same drain/downstack that comes from ensuite (or this pipe is basically directly below en suite).

Today I bunged this outlet to stop any vermin etc getting into it (used a push in cap). I then went for a shower and for the first time had no banging drain. It seems too coincidental not to be related but I can't work out how bunging this drain has stopped a banging drain on top floor.

Don't suppose anybody can explain this to me? My one concern is I'm not sure why this pipework was left open in the first place, there seems to be a vent at roof level

 

[Wayners]

Maybe water hammer?
Have you considered fitting a water hammer arrestor?
Could be lower water pressure so banging has stopped.
You can also turn stop tap down a bit to slow water as that helps stop water hammer if that's the problem