Smoothing return airflow to make fan RPM lower *and* more consistent?

[WhatsThatSound]

Hi again. I'm still working through the "355CAV fan produces variable whining noises that are annoying" problem I previously mentioned but I have a standalone question.

Attached is a drawing of my furnace, sitting on top of its return box or whatever you call it, with its one and only short return coming out the back. Air gets drawn in then up through the fan obviously. On the left is how it was built. On the right is something I tried this weekend. I put a cardboard box in the bottom and propped a board in there to try to steer or smooth the airflow to the fan. So far, this seems to have lowered the reported fan RPM a bit, made the reported fan RPM more steady, *and* made the fan noises significantly less annoying. This seems to make sense to me but I'd like to confirm it. Can shaping the way air flows through that kind of "return box" make a big difference? If so, is there a particular way that works best?

[LibertyTree]

Turning vanes or a scoop in dead-head 90 degree duct turns certainly help the airflow tremendously. I have never experienced what you're describing but if the scoop worked you should have someone come out and put a sheet metal scoop in.

[WhatsThatSound]

Thank you for the feedback. I searched for and found pictures of turning vanes. What is a scoop in dead-head 90 degree duct turn? Would that be like what I did but with a curved 90 degree piece of ductwork?

FWIW, I removed my modifications and the annoying changing pitch fan noises instantly returned. Between your comment and that I'm feeling positive that this might be a way to improve things.

The contractor is coming out this week to go over options. One initially mentioned was to create a second return via 7" flex duct to a new return somewhere else in the home. I think the idea is "more return air, lower fan speed, quieter operation" which does seem to hold in general. Thing is, such a flex could only be tapped into the side of the return box or the side of the furnace itself. So there would be some air coming in via the 14x14" opening in the back of the return box plus some air coming in via 7" round opening somewhere in the side. Two airflows at 90 degree angles so to speak. Having just discovered the important of smooth airflow I wonder if that is one step forward (more air) one step back (more turbulence). I'd welcome feedback on this point if someone cares to comment.