I just had the long block changed in my 300SD,And the motor has less than 6000 miles.But now there is a loud knock in the front of the motor and it seems to not be fireing on one cylinder.Could this be a valve train problem or something different?The motor was installed by a certified MB mechanic and the motor is a new long block from mercedes and is under warranty but the car will not make it to the place I had it installed.Plus I do not have a manual for this car with all the technical data.Can the forum help me?????