hmm i would be looking at your wheel bearings first.

jack up the front of the car. and grab hold of the top and bottom of the wheel. and push and pull the top/bottom to try and feel some play in the wheel bearings. try it from left to right as well.

wheel bearings will generally make noise when you load them up. so if you turn a right corner, the left wheel will make some noise if the wheel bearing is stuffed.

also have a look and make sure your wheel nuts are done up tight. (dont laugh its happened to me!). also your wheels might need balancing, some of the weights may have fallen off.

to be honest i don't think it would be the rack (as ke_70 has stated) vibrations are associated with things that are spinning and are out of balance. though check it out just to make sure there isn't excessive play in it.