Update:
Dealer suspects that it is simply a rattle in the dash and suggested that I leave well enough alone. Alternatively, they offered to pulling the entire dash to investigate but warned that I might end up with even more rattles after doing so.
I’m going to continue to look and hope to resolve it myself. After a bit of searching, I found that owners of the previous generation Outbacks had the same rattle with engine load. It seems the culprit in their case was a transmission ground cable that would rub against the air box in the engine bay. I’m going to do a thorough look at all connection in the Ascent’s bay in hopes of finding something similar. I’ll update if I find it!
