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?