I’m not an HVAC guy but work in the trades. It’s almost certainly expansion and contraction of the metal in the ducts.
Are you off topic for a reason?
or just being a deliberate random?
Cause got to say
you are trying to be underground
but you are floating on the surface
with no purpose in your verses
oh..you are here for swimming lessons?
well here's a tip for ocean sessions
instead of air filled floaties on your arms
fill em with concrete wait till drying starts
then you will swim like a pro
float straight to the bottom yo
that is just for you, i suggest and know