Wondering if this is the brakes or not...

I'm driving down the road and when I hit a bump it sounds like when you put a marble in a soda can and shake it. When I'm driving over the same bump with the brakes applied, it is silent.

Could this be a brake problem? I don't apply the brakes hard, just enough to make the brake lights come on and for the pads to be just touching the surface, so it's not a function of something else being loose when the car is in motion, and then when the car is lurching forward under normal braking it becomes tighter...

I'm replacing my lovely front rotors (75k old...) and am wondering if this is a busted caliper. If so, I'm going to do the whole thing at once. Oh, and before I go throw $200+ down at new rotors, has anyone here had any experience with Zimmerman cross drilled rotors? Are they good? Bad? Ugly? How about Boge Turbo Gas shocks? Yes? No? Maybe?

Thanks

Ryan