I suspect your squeak is the ductwork sheet metal moving up and down along the shaft of the vertical nail or bracket holding it to the bottom of the floor joists.
The 2×12 floor joists on a 16 foot span are well within span limits for single family residences, therefore they are not likely the cause due to excessive deflection. However, the floor joists do typically deflect or slightly sag when you walk on them. Therefore a potentially noisy item, like big metal boxes (ductwork), could create and transmit the sound throughout the house.
I suggest you focus on the ductwork below the squeak and a very minor movement of metal against wood or metal against metal screws or nails. The sheet metal will be better secured, and you will likely have silence. Or you will identify the location and consider removing a section of ductwork to allow access to the squeaky nail, bracket or screw for the repair.
Just think of the whole floor system moving up and down under a normal foot traffic load and what a very slight movement might sound like in the metal ductwork. These squeaks can be very subtle, difficult to find, yet fun to solve.