I belong to a little brick bungalow in the midwest US built in 1954. Original
forced-air (galvanized ductwork) heating, 2-yr-old Heil 80% furnace.

Lying in bed, very quiet, furnace begins heat cycle. About 15 seconds after
the burners fire, I hear a big POP. Very consistently. Every night.

I know that the furnace heat makes the sheet metal expand, and a certain
amount of noise (creaks, squeaks, etc) is to be expected, but this one sounds
too loud.

Walked the length of the ductwork, tapped each joint with a mallet, inspected
everything. No apparent problem.

Still I get the big POP.

Anything with 50-year-old galvanized ductwork that I should be looking for?

Thx,
Peetie