I lurked on this site when I was looking to have a new system installed, and I found some information interesting and helpful. I was hoping that someone might be able to provide a little advice on basically what to ask of my installer. Here's the setup:

I live in a 1300 sq ft condominium in Northern Virginia, first floor, and we had an older Carrier unit replaced. It's somewhat humid and somewhat hot in the summer, so we opted for the 2-ton 19VS A/C paired with a 59TN6 furnace in a closet inside the condominium unit, whatever evaporator coil was appropriate, and the Infinity thermostat. We had it installed about two months ago. The performance is tremendous, and it uses about half as much electricity as the old unit to get the place cooler and more comfortable. Humidity is well under control, at least in the summer. Very happy with that part of it. What I'm less happy with is the noise sometimes coming from what I think is the blower. It's an awful, resonating humming noise that reliably showed up during certain stages of cooling, and not at others. It's the kind of noise that if I heard it on anything else, I'd immediately think a bearing is bad.

The installer came by and definitely identified the noise, but was puzzled by it. He changed the settings from Max cool to Comfort cool, and told me to try to change the latch settings (?) if that could avoid the noise--for example, basically it wouldn't get to stage 5 if that's where the noise was. The end result though, was that the noise moved from stage 2/3 to stage 4 instead, and turning off stages 4 and 5 seems like a bad solution.

My sense is that either the return duct--which runs from the top of the ceiling to the return on the unit about 3 feet off the ground--is causing this or the blower motor is bad. Since then, I've done testing at all permutations of stages / cooling levels / obstructions and list them below, and I can find nothing consistent about this noise. Is it easy for them to swap out the blower or blower motor? I don't know what to insist upon because it seems like something is quite broken, but i'm not sure what. Thanks for any insight. Anyway, if it helps, here are the results of my tests:

Cooling Setting AC Stage Obstruction CFM Static Pressure Noise
Comfort 1 Standard filter 300 0.15 Soft
Comfort 2 Standard filter 386 0.18 None
Comfort 3 Standard filter 468 0.23 None
Comfort 4 Standard filter 580 0.29 Loud
Comfort 5 Standard filter 685 0.35 None
Efficiency 1 Standard filter 585 0.3 Loud
Efficiency 2 Standard filter 650 0.34 None
Efficiency 3 Standard filter 650 0.33 None
Efficiency 4 Standard filter 750 0.37 None
Efficiency 5 Standard filter 825 0.43 Medium
Max 1 Standard filter 585 0.3 Medium
Max 2 Standard filter 675 0.34 None
Max 3 Standard filter 710 0.37 None
Max 4 Standard filter 712 0.35 None
Max 5 Standard filter 793 0.41 None
Comfort 1 No filter 300 0.13 None
Comfort 2 No filter 386 0.15 None
Comfort 3 No filter 468 0.19 Soft
Comfort 4 No filter 581 0.24 None
Comfort 5 No filter 685 0.3 None
Efficiency 1 No filter 585 0.25 Soft
Efficiency 2 No filter 650 0.29 Medium
Efficiency 3 No filter 650 0.29 Medium
Efficiency 4 No filter 750 0.33 None
Efficiency 5 No filter 825 0.41 Soft
Max 1 No filter 585 0.25 Soft
Max 2 No filter 676 0.31 None
Max 3 No filter 712 0.32 None
Max 4 No filter 793 0.38 None
Max 5 No filter 793 0.37 None
Comfort 1 Foam partially covering filter 300 0.16 Soft
Comfort 2 Foam partially covering filter 387 0.2 None
Comfort 3 Foam partially covering filter 469 0.25 None
Comfort 4 Foam partially covering filter 581 0.32 Loud
Comfort 5 Foam partially covering filter 686 0.38 None
Efficiency 1 Foam partially covering filter 585 0.33 Loud
Efficiency 2 Foam partially covering filter 650 0.35 None
Efficiency 3 Foam partially covering filter 650 0.35 None
Efficiency 4 Foam partially covering filter 750 0.41 None
Efficiency 5 Foam partially covering filter 825 0.48 Very Loud
Max 1 Foam partially covering filter 585 0.33 Medium
Max 2 Foam partially covering filter 674 0.37 None
Max 3 Foam partially covering filter 707 0.38 None
Max 4 Foam partially covering filter 789 0.44 None
Max 5 Foam partially covering filter 850 0.5 Loud
Comfort 1 Furnace cover open 300 0.14 None
Comfort 2 Furnace cover open 386 0.15 None
Comfort 3 Furnace cover open 468 0.2 Soft
Comfort 4 Furnace cover open 581 0.25 None
Comfort 5 Furnace cover open 685 0.32 None
Efficiency 1 Furnace cover open 585 0.25 None
Efficiency 2 Furnace cover open 650 0.27 Loud
Efficiency 3 Furnace cover open 650 0.27 Medium
Efficiency 4 Furnace cover open 750 0.28 None
Efficiency 5 Furnace cover open 824 0.35 None
Max 1 Furnace cover open 585 0.25 None
Max 2 Furnace cover open 674 0.27 None
Max 3 Furnace cover open 707 0.31 None
Max 4 Furnace cover open 789 0.32 None
Max 5 Furnace cover open 850 0.35 None