"Could be many things, but from my experience, brake pads might be worn thru and are scraping metal to metal. If so brake pads need replacing. If brake pads are worn thru and not replaced, scoring damage could be accumulating on the brake disc."
It could also be due to a warped or damaged router that would be the big metal disk. but if replacing your brake pads doesn't fix the problem replacing your router will more then likely take care of the noise.
Check the brake pads. The most common cause of the noise is the pad backer grinding against the brake rotor.
Loud
Damping makes a sound weak and amplifying makes it strong.
because the gears are grinding together if its an old car it will make a louder crack sound
More vibrations....
A 10dB (decibel) increase makes a sound that is perceived to be twice as loud.
it makes a loud squeak
it sounds like the belt is breaking
Quantitative
loud and very noisey that they sound like they are screaming.
Geese
When you bang something soft the sound will be soft and when you bang something hard the sound will be loud.The same with talking, if you talk loudly the sound will be loud and when you talk quietly the sound will be soft.