Hello. I too have a very similar audible issue with my 2013 C-MAX. I have approx. 243K on the car and this issue started up about a year ago. The noise as described is very similar. It only happens when the engine runs and the pitch of the sound does change as the engine revs up. I too feel it is something Generator related due to the conditions of when it occurs... I hear a somewhat similar sound when breaking but not to the same volume... More of a whining sound.
I have been told by a separate mechanic that my transmission would need major servicing at some point in the near future given the mileage and the fact that it is sealed. Not sure if $5K + would be worth it but other than this issue I have only had 1 problem with a throttle body that went out at about 96k and it was covered by warranty. The rest of servicing as just been general maintenance and replacing of shocks/struts at a little of the 204k mile mark. It has been an excellent vehicle for me otherwise.
