2010 Ford Expedition EL grinding noise

I have a 2 Wheel Dr. ford expedition EL Limited 2010. When the car is warm, and at speed of 40 mph, when you step on the Accelerator there is a loud grinding noise under the hood . Any ideas? I took it to a transmission company and they told me that it is definitely not the transmission.

Well, that’s good. Could it be a heat shield around the exhaust system, vibrating at a certain RPM?