2000 Buick Century sedan. When braking at 50-70mph, car vibrating brake chatter, frequency slows as the vehicle slows. When braking at slower speeds it is fine. I'm assuming it is either the rotor or brake caliper related since it only occurs when I'm braking. Started after I had the bearings in the left rear wheel replaced. A warped rotor? Loose lugs? Bad Tie rod? CV joint?