The rear disc brake on my bicycle makes noise only at very slow speeds, as when coming to a stop. It is a much deeper sound than the usual squeal heard at higher speeds. Don't know exactly what this issue is called. Maybe judder?
The front brake does not make noise.

Original pads were semi-metalic. Current pads are resin.
Pads don't rub and retract as expected. Pad alignment has been adjusted and seems to be OK.
Rotors cleaned, pads resurfaced.

What are the other possible causes of this issue?

Kyle
Trek FX Sport 4
Stock Promax DSK-927 hydraulic disc
Shimano XT rotors