Issues are more typical with the 2.8 engines - timing chain etc. My '13 x3 3.5, now at 45K miles, has never given me any issues other than a rattling moonroof (common) and the glovebox door separating, both of which were repaired at no cost to me. These 3L sixes are solid as rocks. If you are covered under warranty, great (I'm assuming you haven't put any money into the car. You say you love your car - with that issue and the time/effort you've put into it to try to resolve it, I'm not sure I would. If the problem can't be found, personally, I would trade the car on something else. If you've begun to pour money into it, the decision should be a no-brainer.