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Sounds like the death rattle of a subaru!!! Wud be 99% certain its a big end bearing gone bud.
Engine out and strip down, never what you want to hear but its gotta be done.
Sorry to break the bad news.
Ive had that sound in the past, Not good a rebuild would be required, The more you drive it the more damage it will do, possibly gonna need crank, bearings, Oil pump, Oil modine, new gaskets etc.....