Gas Heating Noise from Attic

Hello,

Is their one or two tanks in the attic one big and one small.

The big tank should not be affected by the heating in any way.

Have you oil, gas, solid fuel heating or perhaps combined ?

Have you a pressurized hot and cold water system ?

Ok the red tank in attic seems like a secondary pressure/expansion vessel for your new gas boiler. The small tank should not be use unless you have another heat source back boiler/stove ?

You need to check with your installer and ask is the small tank in the attic is no longer in use. Is the red vessel for the new gas boiler. Most likely this is the setup but you should confirm.

The switch in the hot press for your hot water does it control the gas boiler to heat your hot water or the immersion heater ?

Do you have anywhere switches to separate your radiators from your hot water when you turn on your gas boiler?

Do you have any zone/motorized valves ?

I'm wondering is it a so-called semi-sealed system fed off the cwst. Non-return valve failure. Air in system. Boiler fires, pump on, air moves back though cwst & alters level of cwst & thus moves the float valve to allow it to temporary fill.

I was thinking along those lines. I do not think a semi sealed system would allow the gas boiler to fire. Unless it is open vented or their is a setting on the gas boiler to allow it to fire on a lower pressure.
If someone was to install like that it is normally to hide a leak.