Hi just joined up so hoping some of you will take pity…
my m665 slx (rear)brakes about 18months old screaming like crazy at all speeds, although it moves people and dogs quickly its doing my head in. I have just replaced pads although not completely worn, sorted for 2-3 rides now screaming again. any ideas please?

Dirty rotors? Give the rotors a good clean – meths works fine or use a dedicated disc cleaner. Check that all fastenings are tight and try again.

What type of pad are you using, and where/ what riding conditions. I’ve had sintered pads squeal like a speared pig but once changed to semi- organic or Kevlar, i.e. a softer compound all the noise went 🙂
And quiet is a good thing 🙂

there seems to be no rhyme or reason to it, altho it’s generally the norm in the wet for most brakes. I once had xt brakes that didn’t scream in the wet, but then started doing it after changing the rotors and/or pads one time. Then i never managed to stop them doing it. all my current brakes screech in the wet, 2012 XT, 2008 Magura Marta and 2007 xt. you might have to live with it.
changing the pad compound might be a solution, but then there are other trade offs.

remove pads and press the lever gently to move the pistons, is there any noticeable bubbling indicating a leaking piston?