Should’ve taken a picture but didn’t want to get in the way of the paid help!!!
Anyway, visualize the square opening with a piece of fiberboard, as wide as the opening is deep, running from corner to corner. He attached, with wide sticky tape, a narrow triangular piece of foam, long side to the fiberboard/short side to the box perimeter. (I’m assuming the foam was included with the new unit). The additional piece of foam added strength & rigidity to that piece of fiberboard that divides the intake from the outflow. He said it was likely acting like a reed in the wind causing a vibration.
Certainly an improvement.
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