I just noticed this, so sorry if a slow response. Anyway, the driver seat had a very subtle "rock" to it (back and forth) and the dealer attributed it to a loose bolt. It was tightened, but no started again shortly after. They tightened it again at the next service interval and, of course, it worked for about a month. I gave up as other people who drove the car couldn't feel it. It never got worse and I grew to ignore it.