Mysterious Metal Plate on Basement Ceiling?
This appears that they used the channel between studs as a return air duct.  Look at the duct that attaches to it and see if it is also a return air duct or if it goes to the return side of the air handler.

That said, drilling a small hole to hang something should be fine.  Make sure you drill as close to the center of the stud as you can - you don't want a random hole in the return air duct if you can avoid it.  If you have some kind of hole-drilling accident, a piece of aluminum duct tape or duct sealant should fix it.