My 99 had he same loud rattling at idle and nobody could tell me what it was. One Ford dealer told me it was one of the injectors "cackling" and there was a newer version they could swap it out for that would eliminate the noise. I didn't and several months later the transmision started shifting bad, slipping between shifts. I took it to a different dealer that works only on the F250 and larger trucks (RVs, buses, etc). The torque convertor was coming apart and the tranmission had metal pieces in it requiring a rebuild. I would get it out of there before it causes more damage.