Much like the Ford Edge with all wheel drive, the Ford Fusion AWD version uses a PTU to transfer power to the front and rear wheels. 

Drivers may notice a slight burning smell, coupled with the distinct brown-colored oil on the ground where their vehicle is parked. Also, a humming, and eventually grinding, noise may be heard as well. 

 

Replacement of the PTU is the solution to this issue, but, be advised, this is a costly component and repair.