I think the answer is in the thread that Anita posted .........
.
my guess is the issue is caused by a vacuum leak .........
.
hose ....... fitting ......... gasket(s) ....... o ring(s) .....
.
especially if it happens at cold start up ........... and then goes away ...... (think Space Shuttle and O Rings) ........
.
I believe this was more of an issue with the early (1999) VX engines .......... there may be a TSB on this ......
.
I do know this same type of problem was common with the optional SOHC Ford 4.0 engines fitted in the Ford Explorers in 97 and 98 ....... faulty/leaking intake manifold o rings were the culprit ...... mine had to be changed two times .....