It is a whistler - not terrible but slightly annoying. When we moved in, we never fired up the old furnace because of manifold cracks, so I don't know if it was also noisy. Before installing the new furnace, the HVAC guy said the ducting should be fine. A lower fan speed works, but it takes forever to heat the house.

The duct opens from below into a larger box. For some reason, extra drywall was removed from the top of the hole, which is roughly 22-1/2" x 4-3/4", and an extra 4 or 5 inches from the sides of the hole exposing some 2x4s framing the box. The vent cover (31-1/4" x 7-1/4") is about 3/4" deep and mounted against the drywall.

Because the vent is much larger than the hole, I suspect the whistling is caused by air coming through the louvers at the sides that are doing nothing more than covering exposed 2x4s. This air has to turn 90 degrees and travel 4 or 5 inches through a space only about 1-1/2" deep before entering the duct box. I can easily open the hole maybe another 3/4" at one side (probably not much help?), but more than this in either dimension would involve cutting into some 2x4s.

Would it help to get a smaller vent, closer to the size of the hole - replacing some of the missing drywall? Would a wooden vent help the noise? Maybe an egg crate vent with bigger holes? Or all of these? Or none of these?

Any and all suggestions are greatly appreciated.