When driving on a road with one half inch of snow, the wheel rims get packed with snow. It appears to me the snow swirls around the disk brake area, then the moisture is sprayed on to the inner circumference of the rims. This happens mostly on the rear wheels but have it happen on the front rims also. Once the rims get full, a chunk of this snow ice mixture falls out now causing a terrible vibration. I have had to replace tail light bulbs because of the excessive vibration breaking the filaments in the bulbs. I have had to stop and clean the rims out with a wooden stick. I cannot believe you should have to do this with today's technology! GM has been notified about this situation but I have not been given solutions to solve the problem.