From much appreciated replies to previous post I'm saddled with an oversized HP, 5Ton vs 4Ton. The noise is extreme, dealer over his head. It has been suggested to add/enlarge ducting to reduce noise. House has 16 - 2 X 12 registers for supply. They are already 17% undersized compared to area of 6" ducts. Am I correct in assuming increased duct diameter to exisiting registers will be ineffective? Removing registers does not affect noise. Restriction ofcourse continues in the ducting. Enlarging registers requires hacking up ceramic tile floors in some areas.
Also noteworthy: I planned on removing an out-of-use humidifier to leave a 12" x 17" hole in a 8 x 24 main duct 2 feet from air handler. I was going to slide a new section of metal over the hole until the noise returned. The idea was to determine requirements for new duct dimensions to reduce the ESP. Leaving hole totally open did not appreciably change duct noise elswhere in house. I assumed that if over pressure was the problem, such a gaping hole would have reduce pressure. Is my noise more than likely fan resonance?