My car is a 2014 MKS Ecoboost with about 61k miles on it. When the car is up to temperature, and I have the A/C going and I'm stopped while in drive (or S) there is a constant humming type vibration that can be felt throughout the car. It feels kind of like the idle is too low or something. If I shift to neutral it goes away. It also goes away as soon as the car starts moving again. If I have the heat running and it's in the evening the vibration does not occur. Mostly I want to know if this is normal or common or what. To me it feels way too noticeable to be just normal operating vibration. I just bought it recently, so I'm still getting used to what is normal on these cars.
I would have said torque/engine/trans mounts first when you mentioned the vibration. Belts/A/C compressor is another.
I don't think it is the PTU/RDU due to the fact that it goes away when the car is in motion. The driveshaft tho, is a possibility, because it is disengaged in P and N, but engaged in all other modes. If the vibration is in the floorboard, that is a good indicator.