Eric, what you are seeing is condensation created inside the exhaust system when it gets rapidly cooled from the undercarriage getting soaked with water. Condensation will form inside the exhaust from rapid cooling and then reheating will blow it out. Not to worry, if you had a hole large enough to let that much water in, be assured, you would certainly be hearing it.
1987 BMW 325
[This message has been edited by jeffsr (edited 05-24-2000).]