Well, a ghost in the machine. Sounds like (no pun intended) a vibrating brake pad or..a rotor that is warped. Either will produce funny wavering low frequency vibration. If all else is well, I would have the brakes checked throughly.
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.