I have a 2015 Diesel Promaster. Several months ago it starting making a growling sound under the hood for a few seconds after the ignition was cut off. Then the same growling sound begin while the engine was running after it warmed up. It was caused by an electronic solenoid part #68250450-AA . Apparently it is not an off the shelf item since it took a week to get. If you are having that sound, this could be your problem.