Homeowner here with typical central townhouse system- Electric AC / Gas Furnace.
Last week had Carrier evaporator coil replaced (under warranty). They also added kick start, replaced external leaking plastic compressor valves with brass, cleaned blower wheel.
Since then, the system has been banging, but they want to charge me $ for another diagnostic call, until they can confirm banging is result of service.
Banging is heard most cleary at night, from bedrooms via the return air vent in the hall. However it occurs at all hours. (It doesn't seem to have anything to do with when blower goes on or off.) Last week heard about 30 bangs when the system was "off" at thermostat, and hadn't been on for many hours.
It sounds like banging is located at the basement interior unit and echoing throughout the ductwork? Banging is every second or so for 25 - 40 times then stops. And not heard again for hours. (Reminds me of bang heard before furnace would go in in my childhood home).
I can't imagine it's the delayed ignition I read about, as it happened when system is off.
Any thoughts on possible causes would be appreciated!