Weird feeling through brake pedal

Strange feeling from brakes. Sometimes when i press the brakes when i finally slow to about 5mph i get like 3 or 4 pulsing vibrations through my brake pedal. feels as if my pedal if being struck by a spring. Not only do you feel it but you can hear it, kinda like some1 was to rattle a spring. my front brakes were new when i got the car and the rear brakes i just did. I had assuming since the rear pads were bare it was the metal 2 metal feeling that i felt. I've got new rotrs and pads and still same feeling.

I apologize its just so hard to describe, Nothin to compare it too. Its not abs kicking it, but when i approach a light coming to a stop towards the end of the stop i feel 3 or 4 pulses through my brake pedal, like as if the pedal was being struck by a spring. Surely i can't have an isolated incident, I don't even know where to begin seraching for the cause of the problem, except checking my brake fluid. Like i said i have brand new rotors and pads in the rear (which i had thought were the culprit being i down to metal), and front pads were new when i purchased the car.