If both front tires have minor imbalance, it can cause a small intermittent vibration in the steering wheel. I've had this happen on one of my non-Mercedes cars. The relative position of the two front tires makes the vibration come and go. If one tire is "pushing" just as the other is "pulling" the two working together can generate enough force to be noticable in the steering wheel rim. Drive around a bend in the road and it disappears. The outside tire has travelled farther than the inside, and they are no longer aligned just so. My old Taurus SHO used to do this when both front tires were ~1 ounce or so off. Not enough for either tire to cause a bad vibration on its own, but enough for them to jiggle the steering wheel by working together.