We recently had a new forced air furnace and return installed. The unit works great, but the return (which is in an adjacent closet to the furnace closet) is quite loud. What are some solutions I can explore to reduce the general noise the furnace makes as it is circulating air?
Similar to Mazura's second answer, the Ductwork from the from the return can and should be really long. If you have the room where the ductwork for the return is, make it really long and zigzag it. Mine could be a Direct shot of 10', though I think they installed over 30' of ductwork and it curves all around the place.
You can detach it from either end, add a coupling and then add more duct.
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.