You should be able to turn it down since you said it is variable speed. It probably needs a technician or an expert here to tell you how to do it. Maybe post your airhandler model number to get some advice or wait and make a service call.

The advice given here is never to expect increasing airflow will help with distribution/duct issues. You always have to fix the ductwork and it sounds like you have proven the point.

most of these variable ecm motor will ramp up till static pressure matches the setting, if duct work is too small and cfm is set too high, this motor will consume more energy then what is was designed to do. as last post..put your model and serial up on here.

Thanks for the comments. I am fine asking the technician come make adjustments, I just want to understand what I'm asking for.

The units are Trane 4TWB3030B1000AA / 4TEE3F39A1000AA.

Having an upsized indoor coil (air handler) does not favor humidity removal. Lowering airspeed in cooling will compensate. Seems counter-productive.

CFMs should be set not more than 1000 for a 2 1/2 ton system at the air handler.

IMO

I just experimented by turning on the fan, only (without attempting to cool the house) -- whisper quiet. The noise only comes when the unit is conditioning air. Could I be hearing machinery noises coming up through the ducts, as opposed to air rushing?

Not sure if this is relevant, but the supply plenum is shorter than previously, because the new unit is taller.

Thanks for your input.