Does your HVAC return duct make noises when the air handler starts and stops? Does your return duct “buckle” from the increased pressure when the supply is active? If so, your return duct may be slightly undersized for the rest of the system. This was the case for me after replacing a few components, and after the swap, I could hear the ductwork when the unit kicked on or off. One perk of hanging with contractors on Pro-Follows is that I can get their advice, and fortunately it’s easy to fix a noisy HVAC return duct.
Pro HVAC contractors I spoke with recommended cutting an additional opening in my return duct. Some time ago, we removed a whole-house humidifier that was mounted to the return and covered the opening with sheet metal. After removing the screws and cutting through the mastic, I exchanged the sheet metal for a return vent grille. After that, I waited for the air to cycle on / off, and I was pleased to find that the noise was gone.