Some neighbors can be a real pain. A couple of suggestions:
1. Check the ordinance that they are using as the basis of their lawsuit. Most of the noise ordinances have a maximum Decibel level at a specified distance; eg.: 85 Db at 100 feet. If the ordinance does not have maximum/minimum definitions then it is vague and unenforceable.
2. Have someone with a Decibel meter measure the Db at start-up at various distances (if possible, from your neighbors house) and at the specified distance. Some of the better sound shops will have a meter, or if your city has a noise ordinance, the police will have them, and if you know a local officer he may be willing to take a reading.
3. Take the car to a couple of Ford dealers and let them do a morining cold start and write a letter stating that the car is acting normally, and within spec.
4. Video/sound record your car and a number of others of the same model/year at a constant distance, use a 100 ft. tape measure to assure the same distance from each car i.e.; 75' or 100' what ever the ordinance requires, and if possible, show a decibel meter in the frame.
5. Counter suit your neighbors for your expenses and fees, plus a little extra for harrasment.