I'm looking for opinions on this one. I have an upflow furnace in my garage that sits on top of a big air return "plenum" box constructed from 2x4s and wood sheathing. This is a common set up around here. The box has been sealed to keep garage air from getting sucked into it. This plenum is fed by two returns, one of which is 20x10 duct (about 10 feet long) near the front entryway, and the other is a very short 20x14 duct in the living room. My airflow is only 1100/1300 on heating and 1000 on cooling, so I believe there is plenty of return for that.
The 20x10 does not produce much noise, but the 20x14 "short return" does. It's more of a low frequency nosie like maybe the blower noise (and echos inside the hollow box), rather than the noise you get from high velocity airflow. The grilles are the ones with the larger spacing and are not noisy at all. I think the problem is that the 20x14 return is very short and feeds directly into the box.
I'm looking for the best way to reduce the noise from the 20x14 return. The big air return box is 18H x 29W x 96L. The 20x10 duct feeds one end through about 10ft of duct, and the 20x14 feeds the other end almost directly.
My regular HVAC guy said he could add some turns inside the box to lengthen the return air path. I don't think there is enough room inside there to create separate air pathways, so what he would be doing is adding some partial "walls" inside there to make the air make a couple turns. I'm concerned that adding those inside the box to lengthen the air path might actually create more air noise because the air has to go around them. I have another HVAC guy who says he would line the box with duct board and that would work best. So who is right?
Thoughts and opinions?
ETA: Oh, and I forgot to mention I have a concern about using the duct board as it is made of fiberglass, right?