What is making a rattling sound when I brake?

What I know:
- The sound originates from the front, driver's side of the
car.
- The sound is rythmic, in-time with the spinning of the
wheels (e.g. as the car slows down so does the frequency
of the noise).
- The sound occurs when I got the brakes at speeds higher
than 30mph.
- I have replaced the outer tie rod, the struts, the caliper, and
the rotor.
- Everything in the suspension system is solid. Nothing
moves or makes sound when I mess with it.

My Ideas:
- Since it only makes the noise once the brakes are
engaged, but it's not anything involved with the brakes
themselves, it must be something close to the suspension
system.
- Maybe the thing only makes noise when the car is pitched
downward, as engaging the brakes would cause the car to
tip forward.