BNB

I have 3 bikes with hydraulic brakes and each make a rather loud warbling sound on one or both of the brakes. Friends have thrown out ideas - brake pads, misalignment of some sort - but none make much sense to me. My bikes with cable disc brakes don't and never have made this noise. I wondered if it was a small amount of air in the lines but none have "mushy" braking. The brakes work well otherwise. I'm guessing this is not terribly uncommon but ?? Any help appreciated.

Rolla

My disc brakes make a sound you could call a warble, mostly on the front, when applying the brake going downhill at speed; not so much during flat/moderate riding. I assumed it's caused by the slots/holes on the rotor.

Iride01 

Mine only make a brief screeching sound after they get wet.

This sound is while you are braking while on the bike isn't it? I can only imagine your brake pads got contaminated with something. Sometimes you can clean them off with brake pad cleaner from the auto parts store or just plain alcohol. But sometimes you have to replace them. I might try a very light sanding of the pad and rotor though before I resorted to buying new.... unless the old pads and rotor is already out of spec. You do check them don't you?

If you do pull the pads to look at them and the wear isn't even and the pad the same thickness through out, then something else is up and that might be the ultimate cause of your warble noise.

Polaris OBark

SRAM/Avid?

BNB

2 bikes are sram hydraulics. the mountain bike is very old and not sure; it's hanging on my garage ceiling at the moment.

it's definitely not screeching - that happens with moisture on all my disc brakes, hydraulic or cable. hard to believe that all 3 of the bikes that are hydraulic are doing this and my cable actuated are not - contamination would be equally likely on all of the bikes.

I'm going to try to record the sound. my newest (sram axs xplr gravel) is the loudest.

oris

My guess is you're hearing a humming that is fluctuates from high to low, caused by the venting slots in the rotors. Usually under hard braking in the front e.g. on a descent, etc.

If it is, that's normal especially under hard braking or at high speed. Generally if you hang on the brakes and really heat them up, it'll start squealing and rubbing after that until it cools off.

Iride01 

I once had a rim brake bike that started screeching very loud and annoying. Cleaning and nothing helped. And they were new pads. And at that time without the interwebs, choices were few. So I just braked hard every time I had to stop during a ride and 2 maybe 3 months worth of rides later, it stopped screeching. Never had to replace them or bother with them again. Didn't have the bike long enough to wear them out again.