how to make hydraulic disc brakes not sound like a trainwreck

there’s oil between the rotors and pads,

clean the rotors with iso propyl alcohol. wipe it down really well.

the pads are trickier. most people would write them off, but you can burn off the organics. take the pads off first. I just use a blow torch, cook it for 5-10 seconds and it’s good as new. I’ve also tried soaking it alcohol and lighting it on fire. The problem is alcohol is not particularly good at dissolving organics so it may take several tries.

If you suspect contamination bathe pads in acetone for a night ( sealed glass / metal container ) and wipe the rotor. Then going over them with >100 grit sandpaper can temporarily help too.

But it might just be resonant combo + flex in components. Without access to parts you can simply add / remove weight and position of various elements – shift the caliper back and forth if possible, re-center it, try different bolt tension ( +/- 1nm of rated ), extra washers or nuts etc to try to break the resonance. Usually it’s the rotor making sound so weight, dampening or different contact point can de-tune it.

But normally one would try *different* pads, different rotor – other brand or type, not just same new ones ( e.g. resin vs sintered or with heatsink, float vs solid, 160 vs 180mm ). Different caliper mounting parts – start with cheaper ( e.g. bolts, then post-mount adapter etc). Maybe you can exchange some parts between front and rear.

Remove pads and sand it down but chances are it might be deeper than that..new pads are probably B05S and about 10 bucks. Then clean the rotor and caliper really well with different rags and put it back together. When the wheel and pads are out do not pull the brake lever, as the pads and rotors are the only things stopping the pistons falling out of the caliper