Here is a nice duct sample. What was going to be a simple system changeout ended up revealing this:
This was caused by an unsealed pvc chase which contains the refrigerant lines, drain, and low vaoltage wire. Common in our area for lineset chases in slab on grade locations.
The rampant, uncontrolled introduction of moist, humid air into the return is a common source for nastiness in supply duct systems.
We provided a solution though... A complete supply and return duct system to go along with a new HP system. The pvc chase did get a nice sealing to eliminate the moisture that created all that extra work.