What would cause a humming noise at 40 mph in a 97 Sable with 103000 miles on it?

Having the same issue with my 95 Lumina. First determine if its on the left or right side, then determine if its the wheel bearing or a bearing in the drive axle. If theres clunking associated with the humming (like mine) then its most likely the latter. Is cheaper to just replace the entire drive axle too by the way