There is what's called a combination valve that supplies the cold water to your unvented cylinder. That has a Pressure Reducing Valve in it to limit the mains water pressure that enters the cylinder. At the bottom of that valve there is what called a balanced supply outlet. That allows the cold outlets to be supplied with the the same cold water pressure as the hot.
- they all don't look like this but this is a typical combination valve - the outlet at the bottom is normally used as the balanced outlet.
It's to help certain types of taps/showers that mix the hot and cold water directly in the body, to be fed with the same water pressure, just helps them work correctly.
Your mains pressure can be tested using a pressure gauge attached to say an outside tap or a washing machine valve, anything that isn't fed from the balanced supply.
No, I wouldn't say that the banging is due to unbalanced supplies.
It's usually water hammer - the outcome of when a valve closes quickly (is associated with higher water pressure too) and it sends a shockwave back down the pipework, the pipework then resonates especially if it's loose/isn't clipped or touching something and creates a banging/thumping sound.