Drafts from AC Ductwork

Jakek

Happy holidays everyone. I was working in my attic and needed to move one of the flexible ducts on the central AC system so I cut one of the zip-ties holding two sections together. To my surprise, there was a noticeable cool draft coming from the system side. This is a small system contained entirely in the attic with the compressor outside. The system is only a few years old although in the house when I moved in.

My question to the group: How can I prevent drafts? In the rooms the vent dampers are closed. The vent grates are aluminum so the magnetic covers won't work. I could make wood covers for vents or otherwise cover them with something but wondering if there's another place to start.

Thanks.