A problem with the ball joints can cause a creaking noise. There is an updated part to remedy this problem.
Car Problem Reports
Mercedes-Benz ML63 AMG Problem With Ball Joints May Cause Creaking Noise
Mercedes-Benz ML63 AMG Problem
Visitor, 2007 Mercedes-Benz ML63 AMG, 60,500 mi
The noise gets louder when it's cold outside (by cold, I mean Houston cold - which is less than 50 degrees Fahrenheit)