I have modified all-metal residential ducting with noise complaints by replacing the take-offs from the main trunk with flex. Keep it straight, kink free, and seal all joints...works fine and greatly reduces noise.
I have also found rhat straight electric heat and heat pump air handlers are the worst when it comes to all-metal ducting with no internal lining due to the blower being downstream from the coil. Gas furnaces with the coil downstream from the blower are considerably more quiet.
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.