typically, the condition that you described is just what it appears to be - the ductwork rusted thru and is now exposed to the earth below the house.
that can create other problems such as groundwater entering the ductwork (being exposed to moisture on either side of the duct could have rusted it thru meaning groundwater could be the cause and not the effect). possible infiltration of pest into your ductwork. indoor air quality problems. possibly/probably other reasons.
how old is the equipment (the furnace or air handler and the evaporator coil) in the house? how much space is in the attic?
i ask these things as it is common when replacing downflow equipment to install upflow equipment in its place and install new ductwork in the attic doing away with the previous layout.
sometimes what originally was the supply ductwork is used with an upflow unit when it is installed but functioning instead as the return ductwork. this can work well but it's important the ducts are in good shape or can be put into good shape.
there is a process that is available that coats your ductwork and is supposed seal it up and put it back into good shape and keep it that way but i don't have any direct experience with that to say one way or the other.
its important that the person doing the duct inspection be a neutral party and i would think it would be correct for a camera to be ran thru the ducts and displayed on a moniter so everyone gets to see what problems either do or don't exist.
get back with us tomorrow and let us know what the inspector says after looking at the ductwork closer if you have any questions about the findings and/or recommendations being made.
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