Tom Meek

Is the car a regular hybrid or an Energi? What state are you in?

If you have another Ford dealer available I'd try them. I recently bought a second 2016 C-Max Energi with 59k miles and it has a low roar/howl under load my first 16 with the same miles does not. The dealer I use has a Shop Foreman who has a 2017 C-Max as his personal vehicle. We drove the car and he diagnosed it as the pinion in the final drive going bad - the dealer is replacing the transmission. The Energi has an 8/100k warranty on the powertrain including the transmission (in CARB states like California it's 10/150k) so don't accept one opinion. The C-Max should be very quiet if it's right. Good luck.

If you have a Discount Tire/America's Tire near you I suggest you take it there for a Road Force balance. They usually have the most up to date machines and are trained in using them. They can also measure wheel run-out to see if any of the wheels are out of round. Good luck.

If your car is an Energi model, this sounds exactly like what just happened to my 2016. The issue is chafing of the high voltage battery harness which throws a fault error that can shut down the car with little or no warning, potentially very dangerous. The fix is covered under Ford TSB 17-0015. Have the dealer look at that bulletin, which you can also find online. This should be a recall item, not just a TSB. File a complaint with NHTSA.