It is not uncommon for the power steering rack to develop a fluid leak requiring replacement of the rack assembly. Loss of power steering fluid may also cause an abnormal noise and damage to the power steering pump.Google+
Car Problem Reports
Mercedes-Benz ML55 AMG Steering Rack Prone to Leaks
Mercedes-Benz ML55 AMG Problem
Average Mileage: 82,750 mi (10,500 mi - 155,000 mi)
Visitor, 2001 Mercedes-Benz ML55 AMG, 10,500 mi
Lost all steering fluid call Mb told to put any steering fluid in. 45 minutes latter all fluid gone poured more in took to be repaired. Steering rack replaced.