On Dec 11, 5:04*am, Puddin' Man wrote:
Greetings,

Little brick bungalow in the midwest US, built in 1954, poured-concrete
foundation. Standard forced-air HVAC, galvanized duct-work.

New Heil furnace/AC in 03-2010. For years, in the winter, t-stat will
call for heat, and about 20 seconds after the blower kicks in, I hear
a CLUNK from the area of the kitchen vent, kinda like someone smacked the
duct with a board.

Back in the '80's I cut a hole in the duct, added a vent for a basement
bedroom near the kitchen duct. Doubt that is connected. Can hardly hear the
CLUNK in the basement.

I'm guessing the ductwork for the kitchen vent is somehow stressed-out,
pops as it warms up.

Yup. Might need to make some room for the expansion. Could be messy.