Welcome to the world of Dodge. I've had 93 Voyager, have 2000 Grand Caravan, and 99 Ram 1500, and they all do it. I did change the idler pulley on the 93 and it stopped for a couple of months, and have had about 20 assorted pull up next to me at lights, that did it. It is not all, granted, but a sizeable share. Seems to do it worst in cool damp weather, and usually goes away after driving for 15-20 minutes (unless you run through a water puddle. I really think it's the idler pulley bearing, and wish they made one that you could service (grease lightly).