Grinding noise under car

I have a 06 RAV4 that is making grinding noises somewhere in the body around 40 mph, under light throttle, almost on deacceleration. It never happens at lower or higher speeds. I did hit a large pot hole about two weeks before this started, but have been told that this has nothing to do with it by Firestone and Toyota. Toyota replaced the 4WD computer assembly, and both places have checked alignment, struts, wheels, etc. Does anyone have any idea what we should check now?

thanks for any suggestions.

Maureen