I had a 2004 Freestar that did something similar. When ever you would start the vehicle it would make a griding noise if the motor was cold or if you just turned on the AC or defrost and the compressor turned on. It seemed to do it the hardest if the van had been sitting for a day or so. Took it to 2 different Ford dealers before they could figure it out. The last Ford Dealership gave me a letter from Ford stating that it had something to do with the temperature of the compressor and the temperature inside or outside of the vehicle (I don't remember). They said to turn off the heeting and AC before shutting off the engine and wait to turn it on until the engine has run for a little bit. This seemed to make it less harsh and less frequent. I wish I still had the letter but I don't and I only remember parts of what it said. Anyway I put over 80,000 miles on it before I got rid of it and did not have any problems with the van. I traded for a 2005 Freestar and it does the same thing off and on. Hope this helps, sorry I don't still have the letter but depite what alot of people say I have had 3 Ford Minivans Windstar/Freestar and have had no problems with any of them except for a power seat motor quiting on the windstar.
