I have a friend who has a 2014 Mazda CX-9 AWD GT. I test drove the car for him with a mechanic friend as well, to see if we could help identify the cause of a strange sound.

He had previously taken the vehicle to a Mazda dealer who said they could find nothing wrong or it was "normal" on the AWD vehicles.

The sound seems to come from the rear differential area, and can be heard inside the vehicle the same with windows closed or open. It is a muffled deep stuttering / growling sound (more "creaking", not "grinding") that lasts only less than a second, and ONLY when accelerating forward at low speed from a complete stop while making either a left or right turn. Such as sitting at a stop light and pulling out to turn left or right. The sound is not loud and is only present for less than a second, under 10 MPH or probably under 5 MPH. No such noises were noticed at highway or city street speeds, or turning in general, only when starting from a complete stop.

I did not notice the sound when pulling straight forward, only turning from a stop. Disabling Traction Control System (TCS) via button made no difference. Sound does not always occur on turns, but maybe 80% of the time.

The steering seems tight and there does not seem to be any play in suspension, or shifting irregularity in the transmission. It does not feel like transmission slippage, but something in the rear, perhaps the differential or multi-plate clutch unit.

He purchased the car with about 45k miles and it now has about 58k. His wife says she has always noticed the sound since they bought it, and it does not seem to get worse although it bothers her, and that is why they have tried to investigate it.

I am aware of PTU (Power Transfer Unit / Transfer Case) issues on 2008-2016 CX-9s, which can go bad and fail catastrophically, due to thermal breakdown of the fluid due to nearby engine, trans, and catalytic converter. The popular preventative seems to be to change PTU fluid every 30k miles. I am suspicious of the PTU, but it is located near the transaxle and engine, not as far back as where the sound seems to be originating from. No visible fluid leaks under vehicle.

Any ideas?

Random tidbit of info from a stranger online:

“My understanding is that the rear wheels are driven by a PTO off the transmission, which is the same transmission used for the FWD models. Essentially, a clutch engages when the difference in speed between the transmission's PTO output (which is dictated by the speed of the front wheels) and the rear axle exceeds a certain point.”

From Mazda's description of the powertrain:

“Front-wheel drive (FWD) is standard on all trim levels with Mazda’s Active Torque All-Wheel-Drive (AWD) available as an option.  This AWD system rapidly adjusts to changing traction needs by monitoring wheel slippage, steering angle, yaw rate, lateral acceleration and available driveline torque.  In normal driving situations, 100 percent of the driving torque is delivered to the front wheels.  During aggressive acceleration, or when one front wheel is on the verge of slipping, a controlled percentage of the available torque is directed to the rear axle. A power take-off device integrated with the vehicle’s transaxle spins a drive shaft attached to an active coupling built into the rear differential.  Commanded by a control module, the servo-controlled multi-plate coupling can route from zero and up to 50 percent of the available torque to the rear wheels.”

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