Rear disc brake chatter/squall

[Suburban_Tyler]

What causes the rear disc brakes to chatter/squall. It does it when there is light pressure on the brakes. I'm a pretty casual driver and don't stop within the last 10 feet of a stop sign. So when I apply the brakes the reach a certain pressure and it sounds like a wallraces getting it on. As I apply more pressure it goes away. If I apply the brakes for slowing down it does the chatter/squall when I apply and release the brakes. I've tried different pads and and I cleaned and cleaned the rotors. It seems to mainly do it at 30mph and below. Seems as if theres enough residual pressure after releasing the brake pedal that 5mph below I can hear a faint chatter squall. The setup I have is a 12 bolt in the rear of my Suburban with The Right Stuff rear disc conversion with Cadillac calipers. I don't have the parking brake cables hooked up yet. The rotors are new of course. I'm not sure what to do or where to start outside of new pads which I've already done. Should I turn the rotors for some reason? The machining marks are about worn off. I thought maybe it was from that. I've never had this issue before.