I had the same thing on my 99. I have since cleaned the Mass Air Flow sensor in the aircleaner with brake cleaner and also replaced the left valve cover & pvc as indicated in the TSB listed below. It's been over a month with 800+ miles and no return of the codes. But still have a constant pinging problem.
The information on the following page has great detail on the TSB that was issued for this problem.
http://leckemby.net/windstar/windstar01.html