F250 2004 diesel. i cant remember hearing this sound before so i dont know if its normal or not. I can hear a hissing sound at the top of the engine kinda toward the middle. cant hear over the diesel engine exactly where from. It will hiss and then go away, then it will hiss agian for a little and then it will go away and so forth. while its idleing with the hood open.

 

Does not sound normal. I would not think it would be a boost leak because you are not making any boost at this point. I am sure some one will chime in.