This has been driving me crazy lately.
I have developed a shudder/rumble in my rear brake. It seems to come when the brakes are not on full stop. This is not the squeal you get from wet or dirty rotors or pads. This is more low and definitely like a vibration rumble. Frame looks ok. I have relatively new pads and a fresh bleed. The wheel seems to be seated properly. Caliper is tight as is the rotor. It’s there 80% of the time but seems to be only when scrubbing off speed. Once near stopping it goes away. It doesn’t do it at all when in the bike stand. Only when riding so that seems like only when it is under weight