Heater causing a knocking sound in ceiling

[cityofvisioncitizen]

2 nights ago I noticed a knocking sound in my ceiling and couldn't figure out what it was because it is only in the ceiling not too far from where my furnace is located. I have noticed this knocking comes and goes, and it starts about 2 minutes after the furnace clicks on when the thermostat kicks it on and it only lasts for about 1-2 minutes at a time. What could be causing this?

[wingardclocks]

It is most likely the metal duct work expanding and or warping due to the heat change. If it is that then somewhere the duct work is fitted to tight It needs room to expand.