 3.9L Pinging
Another 04 Freestar Owner here.
I have the same pinging problem, and it seems to do it even under the lightest of loads. This has occured ever since I bought the van with 0 miles on it. It does it in all weather conditions. I used to own a 3.8L Windstar which did the same thing.
I'm no expert by any stretch of the imagination, but I have a feeling this may be fairly typical with the ford minivan.
I'm taking the van in for a 20,000 mile service and will mention it to them at that time to see what they say about it. It may just be more of an irritation than it is a problem (hopefully). My "soccer kids" depend on it! ;o)
Other than that I have not had any other problems with the van, and I love it. | 
