The wastegate controls how much boost the turbocharger can produce by limiting the speed that the turbine wheel is allowed to spin. The actuator opens a valve (wastegate) and allows exhaust gasses to bypass the wheel, limiting boost to a preset level. Many earlier turbocharged BMWs use a vacuum-controlled actuator assembly, while later model F chassis and newer vehicles use an electronic wastegate actuator.
If your turbo wastegate is rattling, there is a good chance the actuator has been damaged. Replace it with this VEMO actuator to restore proper wastegate functionality.