Steering wheel shaking at 100-120km/h

So recently i have noticed a noticeable shaking with my steering wheel when im driving on the highway at 100-120km/h. Sometimes when i first get on the highway it doesnt shake but when i drive for about 5 mins, the shaking starts. I just got my wheels balanced and aligned, and its still shaking. I do have after market wheels but never any shaking until now. I also got my wheels and tires checked if they were damaged in anyway and they were cleared. So i ask, what could the problem be? Shocks? Transmission? Im getting it in to BMW on monday, but i want to know what some possibilities it could be.



Thanks