For what it’s worth, my ‘13 2.5i also has this issue - clunking at slow speeds over rough or uneven payment.
It’s rather high-pitched. The noise sounds like two tin cans quickly hitting each other repeatedly and then the sound comes to a quick halt.
The car has done this for as long as I can remember (126k now) but I’ve never had any serious issues, mechanical failures, or parts needing urgent replacement. The car has always been serviced at/inspected by Subaru and in fact, I’ve never done any repairs other than regular maintenance.
Could this be a heat shield issue?