I have a 2004 ML350. It's very clear that when the weather is hot above 80+, I hear/feel very light squeaky noise when the steering wheel is turned (even very slightly). However when the winter comes and/or during cooler weather, the noise doesn't produce. I have gone under the car and asked my daughter to turn the steering wheel left and right, but couldn't even hear the noise outside. This is *NOT* the usual noise you hear turning the wheel to the max.

 

It's probably in the steering column itself, upper or lower bearing. But they are not serviceable.