Is there any way to inspect the inside of the long? Think inside where this picture is.

Bore scope where heat runs through?

Not much you can do but to cut it open. The heater duct goes into an insulated tube that takes up much of the space. Looking inside the tube won't show the inside of the long.

I'd suspect rust inside.....Looks like you have to replace the jack support anyway, Cut it open, have a look, treat what you can and replace the support.