Thanks in advance for the help...I've checked posts for this one but it seems unique because it involves both domestic water as well as my heating water. I'll attach a photo of the system.
When water is shut off anywhere in the house a loud banging is heard with what sounds like an echo. It also seems to happen when the furnace completes a water heating cycle. It seems to be coming from a furnace component. When I use the quick fill on the auto feed valve I hear the banging from somewhere within the components pictured. If I shut off the water supply valve to the auto feed the banging is still heard when water is used in the house. The banging no longer occurs when I close off the RETURN ball valve...it seems to solve all of the problem noises. I obviously can't keep the return to the furnace off. Could it be water pressure (hammer) flowing through the feed valve into the furnace system...the check valve located near the air scoop (due to the echo of the bang) or...???
Triangle tube Solo 110 condensing boiler with a smart 40 indirect water heater. There is an expansion tank attached to the air scoop (not pictured).
Pressure is usually at about 15 PSI.