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At a high speed (about 50km/h, but not sure exactly, as I don't have a speedometer) the disc starting to noise like it is touching the caliper/pad. At low speeds It doesn't noise and it's rotating free - it doesn't touch anything. It's hydraulic discs.
Is it a known issue for disc brakes? Or should one to replace the rotor / caliper?
What should I check for (all components are tighten well)?

Edit: It's front disc 180mm rotor. The noise is when going straight.

2 Edit:
@ Vorac & starnger thanks for comments and answers, but it's looks like it's not the situation:
The noise is starting at some speed, and do not get louder it's the same all the time. The caliper and rotor are well adjusted (few shops checked it). The noise is even if I do not touch the brakes at all.
And it's not about lightly touching the pads - the bike is in use every day, and thou it's not downhill, it supposed to be already gone.
By the way, it's relatively new Merida bike, so the brakes should be in good condition yet.

  • More information may help. Can you please elaborate on "sneezing"? What size are the rotors? Does the noise happen at high speeds when going straight, or when turning? Large discs will often "sing" when you turn or lean. They are large enough and flexible enough that the forces created during such maneuvers will flex them just enough to rub, even in a well adjust brake setup. However If you are just going straight at speed and hearing the noise, I would tend to think it may actually be a wheel/hub think because there generally isn't much going on brake wise then. – Deleted User Sep 30 '14 at 23:24
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    I suspect it is touching somewhere very lightly, and going at high speed puts enough energy into the vibration to make it audible. If it is rubbing the pads, a few hard stops down hill should fix it. – Vorac Oct 1 '14 at 8:24
  • Does very light pressure on the brake level change the noise? – mattnz Apr 30 '15 at 3:21
  • @mattnz the bike isn't by me now, but if i'll can, i'll try this – Alexander Apr 30 '15 at 8:04
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If the disc is not touching (as you insist) then it must be the bearings. Take the disc out or pads out and verify.

A more likely scenario is the the disc is indeed touching. At speed you get a higher frequency of touching and the materials heat up and expand slightly. With the expansion they touch even more and now the touching is audible.

  • Thank you for the answer. I never saw a problem with bearings that will be audible only at high speed (as mentioned, it's about 50km/h). True: I didn't check it - I didn't remove the wheel from it's place. By now the bike is not by me, but if it will be, I'll check it. About heat expanding it's interesting point. Did you have in practice a disc that can touch the pads, but you don't feel it holding the bike on repairing stand, nor when you moving the wheel with your hand? – Alexander Dec 2 '14 at 7:05
  • @Alexander I don't think it is the bearing either but you insist the brakes as not touching. – paparazzo Dec 2 '14 at 20:33
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Check the alignment of the rotor as it passes through the caliper. If it's closer to one pad than the other, try to realign the caliper so that it is centered around the rotor.

Does the noise "fade in" as you accelerate? Does it get louder and louder as you go faster and faster? Or is there some critical speed at which it starts?

Does it go away if you ride the brake very slightly? Sometimes a really mild brake rub will go away if you put just a tiny amount of pressure on the brake. If that's the case, I would just ignore it, it'll probably go away/wear in relatively soon.

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