Yes, you are correct about the larger capacity being the reason for the noise.
When the addition is in place and takes its share of the air, the noise will subside considerably. Meanwhile, I would not recommend closing vents or restricting the air in any way for fear of damage to the fan motor.
The unit is simply oversized and sounds like it. Some things you could do to reduce the noise are, 1. insulate your return air duct inside to lower the fan noise, 2. insulate all your ducts on the outside and only on the inside of the boot that holds the register. The external insulation should be 2" fiberglass with a foil outside, while the internal insulation is more like a pressed fiberglass board 1/2" thick turn the black side in to the opening to help hide the insulation it must be glued to the duct and should be pinned to the duct with a spot weld process.