Well, I had a similar problem that wouldn't go away. The case for me? The fan (I guess engine fan, the big spinning thing under the hood!!!) had worked itself loose and had to be replaced, the bearings were bad or something like that! I was skeptical at first, I think it cause around $150 or soemthing, but when I saw the old fan, I realized that it really was damaged.... I don't know how it happened, but the noise and shaking went away....

How can you tell if it is broken? Just take the fan and feel around the center where it connects, its like a bicycle wheel in that it can be out of true or the bearings make it loose around the hold.... I am probably doing a shizz of a job explaining, but I hope you get the point...