Slight one time rubbing noise in the rear of car

When I start driving my Ford Expedition 2013,I hear a one time slight rubbing/ grinding noise seemingly from the rear when the vehicle reaches 20 km/h. Its not a continuous noise.Please help. Mechanics refuse to declare it as wheel bearing noise.
One of the mechanics advised me to get ATF examined and replaced if necessary. I did so. It was a mess.My transmission mechanic just gave it it a try by changing the ATF and transmission filter.He says my transmission is in superb condition. I just had it computer tested.All is fine.The rubbing click is too low to be heard by some of the mechanics during a test drive.One more thing. It happened every time when I turned my engine off and started it again.I never hear it during the rest of the drive above 20 km/h.Gear shift is smooth.
Thanks to those who responded. I am living in Saudi Arabia.