If it wasn't a hybrid, a bad A/C compressor (bearing etc.) might contribute to engine shaking (or a faulty idle up switch that failed to increase engine idle speed when the compressor was running), however, in the 400h the compressor is electrical, powered by the traction battery. It would not cause engine vibration.
Mine was also making a buzzing noise, but only for the first few minutes upon startup. After a short run time, the noise was reduced (but not completely gone) The dealer (as expected) said it was normal, even though I mentioned that it never made the noise when I first got the vehicle.