It's probably the exaust touching the driveshaft. Only happens when it's wet outside, and only AWD cars. Have a look at the driveshaft above the front muffler. If you see a mark worn into the shaft from the muffler, direct your dealer to tech net note #25-21. He should know about it allready without haveing to "reproduce" the condition. The water/slush cools the exaust, causing it to arc up and rub the drive shaft. The tech note details adding a bracket and bushings to prevent this.