Off hand it sounds like the turbo may be shot. Have a trusted mechanic put an auto stethoscope on the turbo as it is running and he/she can likely diagnose it. It of course makes a little noise even when in perfect running order, but if the bearing are worn the vanes could be rubbing and...
Unless I misunderstand, you are not at all upside down on your Nissan. You owe $12000, I show the vehicle is worth $14000 - $18000. Granted, SUV values are not what they once were, but I do not think it has gotten to point where resale values are 1/2 of book value. And in the grand scheme of...