At the front under the engine cover are the 2 air injection actuators, one of those could have a bad vacuum line, also check the entire length of the line that runs from the throttle body to the solenoid mounted on the driver side fenderwell.
It is not unusual to hear some hissing coming from the little covers on top of the valve covers, this is the MB equivalent of a PCV valve.
The small hoses from those covers also go to the throttle body, inside the covers are quite tiny holes that engine fumes are being sucked through.
( hence the need for a NeedWings 'Oil Catch Can' )