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We call that 'oil canning' it is caused by them using thin metal in the furnace plenum construction and associated ductwork, it doesn't happen with heavier gage metal, and when cross braking the metal to add stiffness.
But, we have the old 'lowest bidder' and the cheapest way of sending units out the door. Sometimes, if it is doing it in a place you can get to, you can screw an angle iron, or aluminum, to the metal to stiffen it up.
When my furnace first started up, there was some wire insulation, or plastic from somewhere during assembly in the furnace that stunk real bad, I tore the thing apart and didn't find any wires touching. It went away after a while.
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