Hey. I have a 2016 Cx-5 that has the same problem. The whining noise is audible around 20 mph to 60 mph. At first, I thought the car got louder due to the aftermarket window deflectors. I took the deflectors off, it still persisted. Then I thought it was the road noise because of the tires getting wore out. I took it to the dealer and they said the same thing. However, I tried to listen to it very carefully and realized that it was not the road noise because the noise is not associated with RPM or road surface. It occurred solely according to the speed of the vehicle. I took it back to the dealer. At first, they denied it. However, I insisted that one of the service adviser heard the noise last time so they had to have somebody else test drove it again. In the end, they admitted that they heard it and would like to look further into the problem. They found the noise coming from the transmission a few days later and ordered a brand new transmission for me. The new transmission was installed and the car is now quite again.
I hope this could help you guys who have the same problem.