It's the snow caught in your rims lol. When you drive in the winter about 5 lbs of snow gets caught inside your rim and turns to hard snow, almost ice sometimes. Having that extra weight in the rims unbalances them. Your only way of knowing is put the car in the warm garage and let it melt. Then take her on the highway right after goign thru ONLY clear roads. Do not go thru any snow or ice chunks at all.