Hi, 
     I have same encounter as you. On the day of installation, they came very late and no enough time to run thorogh test. Two days later, when I have time to test the new MHI aircon, i noticed all 3 indoor unit produce "eeeee" kind of high pitch noise. It is especially annoying in quiet ambient envirenment. I cannot image how one can sleep well with it. My aircon specs: 
MHI System-3 Inverter 
Outdoor: SCM60 
Indoor: SKM28 x 3 
     Is your aircon also from same company xxxx Tech?