Hi
My Shimano SLX rear brake is making a high pitched, constant squealing sound as I ride, which disappears immediately if I apply any pressure on the lever. Its not deafening, like the howl you get from contaminated pads, but its loud enough that I'd like it to stop.
Does anyone know what this might be?
Thanks

One pad just rubbing on the disc? - bit like running your finger round a wine glass edge. Check yer pad alignment and reset, see if there is a wear ridge on the pads, clean everything up and re-assemble....
That faint eeee, eeeee, eeee, has driven me mad on occasion.

Tried oiling it?

You know what they say - the squeaky wheel gets the oil

Could it be tinnitus?

Have you, by any chance, been listening to Meatloaf really loud?

Sounds like your brakepads are rubbing..?
I'd check the alignment of the pads (to the rotor) and if it's an older caliper, I'd take the wheel out, remove the pads, pump the lever a little for the cylinders of the caliper to move out (inward to where the rotor would be), then clean the (once) white cylinders with a cord wetted with isopropyl alcohol by slinging the cord around the cylinders and pulling on it to remove dirt. Once that's done, a drop of mineral oil (the one you use to bleed the brakes) on the cylinders, push them back in with a tyre lever or so, clean any residue inside the caliper with a cloth and alcohol, reinstall the pads and wheel and see if it's any better. This usually helps to mobilise the cylinders and can make quite a difference. When running through that procedure the only critical parts are to not move the cylinders inward too far or else they pop out which gives a nice mess and requires bleeding and to give it at good clean before re-installing the pads (to not contaminate them and the rotor).

Other reasons could be glased pads. Original Shimano pads and pads from other brands tend to make quite some noise once they glased. A mirror like pad braking area and a discoloured rotor is an indication for this. If you're not constantly riding steep stuff and giving the brake a hot and hard time, it will very quickly wear off and the brake power will be back to normal.
A few brands like Swissstop and Brakestuff make pads with a galvanised backplate which, in my experience (90 kg with gear, long, steep Alpine descends often with no pauses), makes quite a difference. Also a larger rotor.
But since I take that they make noise without braking, I'd try the above first.

The SLX is probably the best brake for the money. Absolutely love mine (2 pot) on the everyday/offroad touring bike (4 years of regular use, all season/weather) and use one on my trail hardtail bike which gets to see some seriously steep terrain. Though here I swapped (because the kind of riding I was doing was constantly glasing the pads) the original 2 pot calipers with the former XT 4 pot calipers, larger rotors and my fav pads and wow... great performance for comparatively little money.

Good advice from Alpinum but before you get all technical just try this.

Flip your bike over to prop it up on the saddle and bars.

Loosen the brake caliper bolts. Spin the wheel. Apply the brakes. And with the brake applied then re-tighten the caliper bolts.

In many cases, this will happily recenter the pads to the discs and you can crack a tinnie and by happy with your coolness.

If not, crack another tinnie and stare into the sunset ....

Check your pads haven’t worn out! When they wear right down, the little forked spring between them rubs on the disc and makes it squeal.

TheBrownDog wrote: ↑Tue Aug 03, 2021 3:19 pm Good advice from Alpinum but before you get all technical just try this.

Flip your bike over to prop it up on the saddle and bars.

Loosen the brake caliper bolts. Spin the wheel. Apply the brakes. And with the brake applied then re-tighten the caliper bolts.

In many cases, this will happily recenter the pads to the discs and you can crack a tinnie and by happy with your coolness.

If not, crack another tinnie and stare into the sunset ....

Indeed. My emphasis on checking the alignment was very weak and came with no solution. But yes, first just the above procedure.

Misterg wrote: ↑Tue Aug 03, 2021 5:02 pm Check your pads haven’t worn out! When they wear right down, the little forked spring between them rubs on the disc and makes it squeal.

Another good advice.

Seems that simple realignment was all that was needed.
Thanks all