Brake noise is a fickle thing to diagnose. Our lead brake engineer notes this:
"Noise/Vibration. Noise with disc brakes can be a problem. This is true of all disc brake systems and not just Hayes alone. We've studied noise and vibration extensively as we have experts and equipment in this area from our work on motorcycles. On a bicycle there are many variables that dictate how much noise a disc brake system produces. It's not just the brakes... the frame, wheels, rider weight and location on the bike all play a role. The one common finding was that the brake pad compound will influence noise but not necessarily be the cause of it. Typically high performance pads create more noise, especially when the speeds are low, like riding around a parking lot or on flatter sections of trail.
The way it works is the higher the coefficient of friction (Mu) of the brake pad, the greater probability of noise being an issue. This is due to the specific harmonic resonance of the bike as a system. I like to think of it in terms of a violin, bow, and rosin. The violin obviously has a way of transmitting sound, but only when the bow is dragged across it. But the bow alone, if left unrosined, won’t vibrate the strings uniformly so the sound is muffled. Once you rosin the bow, the friction between the bow and strings becomes greater and more uniform, thus allowing the sound to ring out at greater volume. By rosining the bow, you increase the frequency of the vibrating string. High Mu brake pads behave the same way on a bike. It’s one area I think will start gaining some design attention (eventually) with all bike manufacturers, as through the design process they can help mitigate some, if not most all, the noise issues associated with the brakes. I’ll forgive them if they can’t get a sopping wet brake to run silent. There are only so many things you can do about that particular howl.
So, geekery aside, what can be done here and now about it? Semi metallic pads can help some here, as can full organic pads. Also lower Mu full sintered metallic pads, the same compound as the pads used in the older Mag and Nine series brakes, and these brakes weren’t exactly known for having noise problems. All the aforementioned have their drawbacks however. In the case of the semi metallic and organic pads, they suffer from a shorter pad life and poor performance in wet conditions. Also, the semi metallics also cause more rotor wear, as the suspended metal particles in the pad matrix are larger, much like a coarse sandpaper. The lower Mu sintered pad obviously has a weaker bite to it when compared to our standard sintered pads. This may/may not be an issue as depending on how the bike is used and what conditions it’s ridden in will be the end all determining factor."
How to stop it - here are some tips and tricks.
1. Be sure to properly burnish your system. See here for details.
2. Be sure ALL hardware including the brake calipers are tightened to their proper torque values. See here for install video.
3. If you are hearing a howl or what sounds like a loud horn noise.
That description is what typically happens when a frames natural resonant frequency matches a brake frequency. That is why it is so loud, the entire frame is the speaker. Sometimes you can get past this by using a different rotor size which may move the frequency enough apart from the frame. Sometimes using a different pad compound will work. We suggest trying the other pad compound first. Then switch up or down a rotor size.