Front disc brake rattling noise

Question

I recently purchased a brand new Scott Genius 950 at a local bike store. After bedding in the brakes, I noticed a rattling noise while riding coming from the front disc brake. I brought it back to the bike shop, they disassembled the front brake and put it back together, but the noise is still there.

I was told to bring the bike back when the head mechanic is in, but I thought it would be worth posting a video of the noise here. It seems to act up the most when I turn sharply left or right. It doesn't always rattle, and it does it when I am not braking.

Maybe it isn't the brake, but the spokes? I am not really sure.

Any suggestions?

UPDATE 1

I just got back from the shop, the head mechanic took the front brake apart and put it back together. I rode it around the parking lot for a while, and just now around the neighborhood. Seems that the noise is gone (for now!).

I asked what he did, but I don't remember all of the technical details. Definitely re-aligned everything, tightened everything, mentioned something about [retracting?] the spring, etc. I wish I remembered more of what he said so I could share it here.

UPDATE 2

I just got back from a test ride around the neighborhood. I can still get a slight rattle out of it when dropping the front end after doing a wheelie. Since the front brake has been rebuilt by the shop mechanic, I am not too worried about safety. From the feel of it, nothing is loose, and I have full stopping power. I suppose I will just have to learn about it on my own, and continue to investigate.

UPDATE 3

So, I am able to reproduce the noise (only for a fraction of a second) when dropping the front end down, hitting a curb at an angle, and turning sharply. It makes a very quick, very quiet ching noise. From this, I am now thinking that it may be from the flexibility of the fork causing the rotor to make brief contact with the pads. After learning how the disc brakes on my bike really work, I am not concerned that this will be a safety issue, and may wear in with time if my conjecture is correct.

I found some interesting information after looking at the dealer's manual for my brakes: SHIMANO Hydraulic Disc Brake 4-Piston Caliper Model: BR-MT420

https://si.shimano.com/pdfs/dm/DM-MBBR001-04-ENG.pdf

On Page 8 about a third of the way down the page it reads "The 203 mm and 180 mm disc brake rotors have a larger diameter than the 160 mm disc brake rotor for cross-country bicycles, and so the flexing of these disc brake rotors is greater. As a result, they will interfere with the brake pads.".

Very interesting, as those are the exact size of the rotors on my bike. So maybe this is normal and expected?

Thank you to everyone who has responded, and if anyone has any further ideas or experience with this issue, please reply :)

Answer 1

The rotor might not be perfectly true. Although the shop should fix it as it's a new bike, do be aware that big MTB rotors get out of true really easily and at some point you just need to live with it. This can be somewhat mitigated with two-piece rotors utilizing a beefier spider, but in the end you're riding trails; stuff breaks all the time.

Is your thru axle tight? I know that with 9mm QR forks, violently turning and braking can sometimes be enough torque to momentarily twist the hub/dropout interface out of alignment, resulting in noise. A similar effect might manifest if your TA is not done up properly.

Answer 2

That noise is a concern. It certainly sounds like something is hitting the rotor, but if the rotor is out of true you should get a periodic 'ching-ching-ching' sound. The noise is more random which makes me think something is loose, possibly the pad retraction spring has broken or come loose which is allowing a pad to rattle onto the rotor. Personally I'd avoid riding the bike until it is looked at by your bike shop. I'd be worried the pad might jam the rotor.

Answer 3

I have the exact same noise on a Cube Stereo Hybrid Race 500. It has the same model of brakes (Shimano BR-MT420) and 203mm rotors. Edit: The noise occurs most often at speeds above 20km/h and especially when turning. Sometimes also even when going straight.

For me the noise comes from the rear end. My bike is new with only about 180km on the clock. I took the bike back to the shop for a check-up and this is what they told me after making sure everything is aligned and there is no play in the hub.

1. The noise is caused by the rotor touching the pads. The reason the noise sounds random and not periodic is because air escapes from the holes in the disc, causing vibration.
2. The rotor, being fairly thin and cut out from a single sheet of metal, is almost like an instrument. If you flick it with your finger it will "ring".
3. The noise is normal and should fade with use. (Not clear to me why it would fade)
4. After alignment in the shop the noise was mostly gone but they told me it would likely return after some heavy braking.
5. To get rid of the noise once and for all they said I could upgrade to floating Galfer rotors. I suppose other sturdier/floating rotors would also do the trick.

Answer 4

Ive experience similar issue when testing ou my mates bike multiple times on different bikes, honestly I still don't know much about why its there, but when I change to another different rotor is noise is completely gone, so I believe the orientation could be wrong before. Dont worry about stuff is loose I literally tested myself to make my brake rotor and caliber very loose and brake hard and there is no noise at all,