How to address loud cold air return

Question

The cold air return on my forced air furnace is too loud. While it is is tolerable, there is a noticeable sound of rushing air when the furnace is running (not a whistling sound, just a blowing sound).

I have a KeepRite G9MVE (80,000BTU 1600CFM).

I have read a couple of other posts an online resources, which generally seem to suggest:

1. liners or baffles
2. less restrictive grill
3. larger duct size

For #2, I have removed the return air grill, but no change.

For #3, I removed the blower door and ran the furnace again. The return air was not noticeably quieter.

Using some of the calculations in the third link above, I think the 24"x6"=144sq.in trunk is probably too small (something like 900fpm flow), but then why would it not be significantly better with the blower door off? Doesn't this effectively bypass the return air duct?

Is there something I am missing here? How can I improve this issue?

My ducting looks something like this:

Here is the cold air return intake (A in diagram - 12"x30"=360sq.in) The intake runs between two pairs of floor joists, and the gap closest to the furnace, left side in photo, is noticeably louder than the other side.

Here are the suspended trunks - return is on right, about 16' long (B in diagram - 24"x6"=144sq.in)

Here is the furnace connection (filter 16"x25"=400sq.in)

Answer 1

I'd have to say get an HVAC guy or firm. My first thought is that they could relocate this one under the stairs & have it suck through the side wall of the stair's stud bay. My 2nd thought is to add the stair stud wall as a 2nd smaller return, but then also move & shrink the floor one away from the stairs if there's another 4 or 6' away it could be.

The idea is that there should be a large sound reduction by getting that return out of the direct line of the HVAC fan's draw. You'd be getting rid of the direct sound path of the fan motor & fan, but also adding distance between you & the fan noise & splitting the suction whoosh in half. Definitely, see what they say. I don't see any of the other options doing anything, just like what you've tried & determined already.

Answer 2

If it's not significantly better with the blower door off, while you cover the grate with a plastic garbage bag or something, what you're hearing is machine noise and IMO only solvable by my answer for #1.

But if the problem is whistling, your options are to lower the blower speed or enlarge the ducts. I see that yours is a newer unit, adjusting the blower speed lower may be an option. In fact, it may have been set high in the first place to push it though your ducts better.

If you do lower the speed you may be interested in my cut the gas back trick, so that it doesn't kick out on over heat.

Read this manual thoroughly and play with the SW1 switches (p. 13) to adjust it and make sure this manual lists your exact model number. It also includes the proper way to reduce the gas flow rate.