It has been dubbed "Subaru Deceleration Noise" since at least 1998. It is nothing new.
Adding a stiff transmission mount will cause you to be able to hear it on a totally stock car. Even without that, many stock cars do it. If you take it to Subaru for warranty, they give you an even heavier flywheel. Going to a light flywheel makes it very, very noticeable. It is not a bad thing. It's not hurting anything. Your flywheel didn't cause it, it only caused you to be able to hear it. Your car has always done it and always will.
Running a higher front/rear tire pressure differential will make the noise get quieter or even go mostly away. You might try a stagger of 5 psi... as in fronts at 36 psi, rears at 31 psi.
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Slow and low, that is the tempo.
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