From what you describe, i think i have the exact same problem. I would be very appreciative to hear the diagnosis of your system. thanks
This has been awhile but the final resolution was that Trane mis-speced the coil for this heat pump. It was too small. They exchanged the coil to one with slightly larger diameter coil lines. It has been running well for the about 18 months both heating and cooling. Very very infrequently I hear a pressure valve give on the outdoor unit, hissing and the pipes vibrate.
Hope that helps. If you need the exact model of the new coil I can dig it up.