When driving on the highway, our '08 4x4 Escape has a very noticeable vibration in the rear end that sounds sort of like a distant helicopter. When we took it in to be looked at, the tire technician said that Escapes are notorious for bad tire wear, which creates this noise. He said he had an Escape that did the same thing, and produced the exact same noise. So is this normal? Any tips to silence the perusing helicopter?