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Give me a few more details .Is the furnace in a closet,attic or basement and do you know if he has adjusted fan settings to match the ac system?
I would make sure the Blue fan wire is on the COOL terminal, This is Goodman's recommendation of medium speed for a 4 ton AC system.If black is connected to COOL terminal, that is high speed and will be a bit noisier than medium.If already using blue on COOL terminal, you can install some duct board baffles in return to help quieten the unit.I have also used ductboard around sides of furnace cabinet to help with noise in certain situations. The GMS is an entry level furnace and Goodman is one of the least expensive units on the market,so a certain amount of quietness as well as quality is sacrificed as compared t say a top of the line Lennox or Trane.