Does anyone else have a passenger seat that shakes when un-occupied? If I'm doing 55 mph or faster and there is no-one in the front passenger seat, the entire backrest of the seat shakes, with the head rest moving +/- 3/8" back and forth. I've searched the forum and found only one other reference to it. I'm thinking that this is not a common problem and I might mention it the next that it I take it in. And yes, I did check to make sure that the seat mounting bolts are tight.