Thanks!
I looked up the stats on the air handler, and it looks like it only has two speeds, and only 3-5% difference between the two of them:
Edit: Pic didn't show up. These are the CFM ratings on High/Low at 0.1", 0.2", 0.3", 0.4", 0.5" H2O
High/Low 1,586/1,524 1,530/1,490 1,470/1,420 1,410/1,370 1,350/1,175
That being the case I'm starting to think it may not even be worth the hassle.
You talk about noise. How is the airflow? Is the noise from the air or the blower?
It's tough to say whether the noise is from the air or the blower. Airflow is strong - I'm assuming stronger than it should be for the size and quantity of ducts. The vents closest to the air handler are definitely the loudest. The vent in the kitchen for example seems to have moderately less air velocity than the vents in the family room, but it produces significantly less noise. I think that could be because they've got more air flowing through them because of shorter duct runs, but it may not be the whole story.
When I added the return vent to the master bedroom, I could feel a good bit of air coming out the other side (3 supply vents in the master suite), but it seems to be a much lower velocity than what comes out of the supply vents, and it makes no noise at all. I intentionally picked the largest return I could fit for just that reason.