Don't take it back to that place again unless you it is a MB dealership. It does sound like a dealership...Am I right? If it is, speak to the service manager and/or the MB service rep for the region. Otherwise, just take it to an independent MB mechanic. May be somebody here can direct you to a good MB mechanic in your area.
It sounds like you have air in your system and the blower fan is out of balance.