'99 Windstar trans

Bought a used 99 Windstar from the Ford dealer with the Basecare warranty two weeks ago and the trans rans started to leak yesterday. It's leaking where the engine and trans meet. We moved the van from park to reverse and heard a loud pop noise and felt the van shake a little. That's when it started to leak. It leaks only when the van is running. Needless to say, it hasn't been moved since. It's going back to the dealer tommorrow and hope it's covered under the warranty. Any idea on what would cause the van to do that when shifting from park to reverse?