'94 deville 4.9 brake noise, pulsate, under 5MPH

This problem is on and off, not sure if warming/driving the car awhile will start it as it did today. Sometimes you drive the car and nothing is wrong. I brake to slow the vehicle at a red light for talk, the pedal is fine until 5MPH and below, this is where the pedal makes a clunk/buzzing noise(like a rrrrrr noise) and pushes the pedal up and down a little (feels like something is literally grabbing the brake pedal and moving it) and the brake actually feels more solid after it does this routine, if that matters.(compared to when it doesn't do it) I think you can hear the NOISE outside the vehicle as I've had people look at me and stare at crosswalks.

Brake observation:

-pads thick in front all 4 sides

-pads in back seem uneven, thick on passenger rear, and slim on driver rear and the outer seems thicker than the inner pad on that side too. Cylinder full, no leaks, booster checks okay, no lights or codes thrown.

side note: I jackstand the entire back, spun the rotors by hand(smoothed a little with emery cloth too) and had a helper push the pedal, both sides stopped immediatly and looked like all pads were pressing correctly except for the emergency brake, this only stopped the passenger wheel not the drivers side, as the wheel still moves when turned by hand(with emerg. brake engaged), is that normal?(no right)



I did notice the driver back rotor was hard to turn by hand when part of it reach the pads, the passenger side turned smoothly, is disc runout my problem 100%?



I had nothing done to any of this system since i bought the car march 1, 05, and it just started this routine a week ago. The car appeared like it was sitting or something before purchasing(before the dealer got it), the trans fluid was nasty too.(replaced)

It sounds like your ABS system is engaging at low speeds. There could be a speed sensor that is going bad. I would check the computer codes, and see if any codes are stored for the ABS system.
Do any lights come on the dash when this happens?