Hot Water Boiler pipe banging during very cold outdoor temp

[ap.]

I have a hot water boiler that causes pipe bang, but only when its very cold outside.

I once lost some water from the system while changing the cir. pump.

The kater valve is functional and the relief valve is fine.

I replaced the old rusted expansion tank with a new diaphram type.

Is air causing this problem?

I had some radiators upstairs turned off.

help.

ap