Kind of a long story, I traded in the 17 for a truck and then about 6 months ago we tried to sell my older 2012 legacy but I couldn't get what I wanted for it, so my wife and I talked and decided to get rid of the truck and bank some money for as long as I could keep the car running. Well, as soon as I sold the truck, that car was nothing but problems, I was working on it every weekend. I got sick of it and traded that car in for this 18.
It's a beautiful car, a limited with only 24000 miles, was apparently kept in a garage because even the underside of it still looks brand new. But I think I see why someone traded it in!
As far as diagnosing, I am pretty mechanically inclined, I do most of my own work. I even changed the timing belt on my 12'. But I have never heard anything like this sound. Whatever it is, it is definitely not right, and I suspect it could be something serious. I did purchase a good warranty, so it will get fixed, eventually.