What is this noise from rear brake disc?

[Jody]

Had a similar noise from the front disc on my Genesis Croix de Fer... Did the same as OP and changed the pads, made no difference. So changed the disc and problem solved. No idea why; maybe it was worn but still looked ok? I don't have vernier calipers to accurately measure disc thickness, so it was just a guess.

It will eventually go when the pads wear to shape. Had it a few times now

 

[middle of the road]

My wife's new Decathalon bike made the most awful screeching noise when the brake was applied. It wasn't getting better and put her off cycling. We returned it to the store and they did take a look, tried to centre the caliper etc, everything I'd already tried. Because it didn't make the noise when on a stand it was declared fixed. The first ride showed that it wasn't!
On our next visit we saw another chap and he said the pads needed to bed in and the discs get polished from use.
By this time the bike had been ridden for about 300 miles.
I'm not sure if it is a permanent fix but I ended up removing the pads (held in magnetically) lightly sanding the surface and applying a smear of copper grease to the back of the pad. Has it cured it? Not entirely, she can still get it to occasionally shriek but heaps better.
Did the light rubbing down, the copper grease fix it or did they eventually wear in? I don't know.
All I can say is rim brakes are so much simpler.

 

[Pedropete]

FWIW, cable routing looks fine. You'd generally expect the hose to run inside the seat and chainstay. As for the noise, few things I'd check but could be any or none of them:

- Did he definitely replace with the correct pads?
- Check the pad spring hasn't been trapped on the face of the bad
- Did he move the caliper before the noise started?
- Check the caliper is correctly aligned and secured in place

 

[Gillstay]

Its the plastic trim thing behind the sprocket, that's why its so loud. Well thats my guess anyhow.

 

[brommieinkorea]

I too think the plastic thingy could be amplifying the noise. But here's what ya' do: First, make sure the pads are correct, and properly seated. Second, make sure the disc is centered and bolted properly to the wheel. Third, check to make sure there are no loose spokes, or cracks in the rim or hub( this is a wheel integrity check for step four). Four, bed in the pads. Get the bike going fairly fast, preferably downhill, and apply the brake (don't skid that tyre it won't help), do this maybe 5 times, yes the disc will get hot ,don't touch ! Let it cool down and then repeat your noise check.
The reason bedding in works to reduce noise is because the former pads would have worn with the disc and so heavy application of the brake puts those same grooves into the new pads. A slight flat sanding(800 grit) of the disc could act as a brake lathe does to restore the smooth finish to the disc. This said I don't even have the discs on my car turned anymore because it's so cheap to buy a new set, likewise a standard 6 bolt brake rotor for a bicycle isn't too much to replace.