Took my brand new 2009 E (under 750 miles on it) on its first highway trip this past weekend. When I hit the highway a few minutes after starting up, it started vibrating/rumbling a bit (as if the tires needed balancing) for about 45 seconds or so. It could be heard and I felt it in the steering wheel. I attributed this (as a guess) to the tires being cold (it was about 35 degrees outside). However, an hour or so later when the tires were clearly warmed up it did the same thing. It didn't seem related to speed. After another 30-60 seconds the vibration slowly disappeared and didn't appear again. Any guesses as to what it might be?