I have a 2002 expedition. It has a sound in the back that sounds like a millions bees or high pitched humming. Kind of hard to explain the sound. Regular mechanic said it was,but he don't do transmissions. I thought it was in the wheel.(just my thought). The faster you go the louder it gets. When you let off gas not as loud. Is there a tool a mechanic can use to test it? Second opinion maybe as to what the problem is? It has 161,000 miles on it