I'm not really familiar with Subarus, but I'm quite certain the service manager is full of it
If his theory was correct, EVERY single engine with a timing chain out there would be rattling on startup, after sitting overnight
BTW . . . overnight is NOT considered "extended period"
It's clear the manager is just plain wrong, an idiot, just doesn't want to deal with you, or one or all of the above
I would not stand for this for one single minute
They are clearly taking advantage of you
I like your idea of writing a letter. Keep records of everything. Who you talked to, when, if they returned your calls, etc. You might have to use it as ammunition soon
Word of advice . . . make that letter a "sign on delivery"
That way they can NOT claim they didn't receive it