Hum Sound

Sounds like may have a differential noise or possibly a bad wheel bearing. They both make different sounds that most mechanics could easily diagnose with just a quick road test. If the noise seems to be on one side or the other it could be a wheel bearing. A bad differential will make more of a gear whine sound that is more audible coming from the center front or rear of your vehicle
You should check your gear fluids immediately to avoid possible further damage and eyeball for leakage at seals at the same time. If you have a standard shift have that checked also. Driving around with an empty gear cases can cause you further problems.