I was really happy to stumble across this thread, and the question posed by JustBlueFish in the o.p. I too bought a new J-45 from Sweetwater (in 2020) and with Elixir strings, if I strum any chord above the 5th fret I get intense metallic resonance across GB&E that kind of sucks. I took it to my tech for a real setup, not what Sweetwater provides, and he improved it somewhat. I will try different strings based on what I am reading in this forum. I did play J-45s before buying mine, and I can only go from memory, but I don't recall hearing this issue.