Here's the juicy bit from a Ford 2012 press release (for the Fusion hybrid, but same applies to C-max):
"At the lower speeds that help efficiency, engines produce booming, low-frequency sounds that can be unpleasant to the driver and passengers. Without Active Noise Cancellation, engineers have to keep the engine out of this operating range, sacrificing efficiency.
Yet to achieve peak fuel economy, a creative solution must be applied harnessing advanced audio technology to eradicate this unwanted sound.
A predictive control system notes driver behavior and how the engine responds, anticipating noise before it is even generated.
Three highly sensitive microphones are mounted in the Fusion Hybrid’s interior headliner – two over the first seating row and one over the rear – to detect and measure engine noise. These microphones send a signal to the Active Noise Cancellation control module in real time."
“The Fusion Hybrid leverages this advanced technology [to help deliver maximum fuel economy for the customer,” said Henry Ford Technical Fellow for NVH (Noise, Vibration and Harshness) Dr. Takeshi Abe. “Reduced powertrain noise in the passenger compartment becomes the icing on the cake.”
Comment: SO the ANC signal fed to the audio system has to be perfectly out of phase with the noise in order to cancel it. If the ANC has a phase that is just a bit off, it will add to the noise rather than reduce it. That appears to be what is happening. Customer can end up with amplified "booming, low-frequency" sound rather than cancelled.
(Looks like (from LInkedIn) Takeshi Abe is a vehicle noise consultant hired by Ford for the Fusion / C-max project, and who then retired right after.)
1. ANC on/off available to user.
2. ANC parameter adjustments available to user or technicians.
3. Option available to consumer to tell PCM to keep engine out of "boom" producing rpm range in cold weather (accepting mpg tradeoff).
4. Better power train isolation - since it is a cold weather problem, seems likely powertrain mounts are losing elasticity and transmitting more bad vibes.
More expensive fix, but perhaps closer to the heart of the problem. Maybe install more deluxe mounts in cars sold in cold weather areas.
5. Stiffen the panel that vibrates, maybe where the low center powertrain mount attaches.
6. Better balancing in engine to reduce the problem at its source. (not a retrofit option)