Not sure what this is, but when I first start my car and drive off, my brake pedal makes this weird noise, as if it pushes itself down or something? It only happens when I first drive off, and then it never comes back. Happens every single time... is it my ABS?

I know my rear right rotor is warped as well, but I'm changing both my rear rotors and pads this week.

Any help would be greatly appreciated....

Thanks