banging from air ducts (furnace, AC, heat, paint)

[nyc2020]

I hear an occasional bang/loud tap coming from inside the walls. I'm pretty sure it's not the pipes but rather the ducts. interestingly it makes the noise even when the heat or ac is not on. what does this mean? will seasonal maintenance on the furnace do anything? the noise has been there all year round since we moved into this house two years ago

[James1202]

Expanding/contracting ductwork, can make an occasional metallic "bang" as it expands and contracts with temperature changes. Wood can make a "loud tap" or "crack" sound for the same reason, as well as, during high humidity or during the process of drying out as it, too, expands and contracts.

During Winter's cold, one might see Summer's hairline cracks, in painted corners, expand and then re-close during Summer. Cracks that don't close, appear elsewhere other than corners, or get progressively wider or longer could indicate settling or some sort of structural failure.

[K'ledgeBldr]

Poltergeist, vermon, or more likely just expanding and contracting sheet metal ductwork.

If you can isolate which duct it is and have access to it, you maybe able to reduce or maybe even eliminate the noise by moving, twisting, or even denting the sheet metal just abit.