Moisture in Attic
As you know, when warm air comes in contact with a cooler surface, the moisture condenses OUT of the warm air onto the cooler surface.
If the ductwork isn't insulated, then what you're experiencing is an eventuality. Yes, usually attics are to be vented. How and how-much, depends. Convection will cause a transfer of heat and cold between surfaces. That could be contributing to your problem depending on which 'side' of the situation is warm, and which side is cooler.
Insulate and leave some 'space' around the ductwork, and you should be fine. Of course, I can't see it from here. My best to ya and hope this helps.
Jay J -Moderator
PS: God Bless America!