To address the rumbling/whistling for our new 2.5 ton Trane air handler, the installer recently added an additional 12" x 12" return duct in a bedroom to complement the 12" x 24" return duct located in the hallway. Is the new duct large enough? While it has helped reduce the rumbling and some of the whistling, it didn't eliminate either. And the new duct makes a strong sucking sound when the unit is running fully.

Would a larger return duct in the bedroom help (12" x 24" rather than 12' x 12")? Or would it help to add a third return duct in another bedroom?

And since a prior poster asked about return duct location, the original 12" x 24" duct is located about four feet from the attic air handler. The new duct is about eight feet further away. Both are located in the ceiling and the air handler is in the attic.

Any help would be appreciated.

Thanks so much.