How do I reduce the noise from my forced air furnace?
Fiberglass Duct Liner: (industrialinsulation.com)




  Fiberglass duct liner is designed to be installed inside sheet metal ductwork and plenums.  Fiberglass duct liner absorbs noise and contributes to indoor comfort by lowering heat loss or gain through duct walls.


Or if you have the space for it, build some baffles out of 2x4' acoustic ceiling tiles inside the return chamber like below. Secure them to some sort of structure so they don't get pulled into the blower.



Using a pleated filter (the white kind) instead of the fiberglass type will choke the air flow a bit more and probably cut some of the whistling noise.