Greetings krwendel welcome to the Forum.
Yea, those old units will do that, and other than the noise, it's not much to worry about.
When the Duct was new, it probably had "Cross-breaks" in the flat surfaces.
That's where they bend the larger flat surfaces, where they are more rigid, which helps with the structural integrity of the unit.
As time passes, we often compress these "humps" in the metal (sit/step on um) making them flatter, and the temperature variation will make them pop.
There's not much you can do about it, but, I've seen one serous instance, where the user opened up the Duct, and put a Board in it, pushing the upper surface upward, like in tension. That may be an option for you.
Have a good Day !
S.W.
PS: EDIT
I must say that the Board was NOT placed in the Furnace chamber, but down the line a ways.
We are looking for volunteers to join the moderation team.
more posts...