It is not uncommon for discs including car discs after changing pads for the disc to squeel.
Because the disc will inevitably wear with use, the old brake pad that causes the wear will also be worn to fit perfectly to the wear on the disc as at engages, whereby grooves in the disc that are formed will be identically paired to that on the brake pad.
Squeeling is also common for old pads, as well as poor quality badly manufactured cheap pads, or excessive use of brake pads - where they get burnt and are too hard against the disc or dust fibres from the brake pad get caught within pad and disc. It could even be a combination of these that causes squeeling.
When you change the pads which are now new and being smooth and unworn they do not mate up anymore with the groove marks created from the wear of the old pads. The discs when brake pressure is applied will always vibrate to some extent, even on new bikes, however if imperfections exist such as the fact there is no wear marks from new pads rubbing against the old disc with its worn imperfections or even improper fitting etc occurs - they will sing in a high pitched squeel at certain speeds, even with good quality pads.
Other than ensuring all items are set up correctly, get a good quality brake pad and if it continues squeeling consider replacing the disc rotar itself or get the old one machined by a mechanic shop to be smooth again.
Thinking more about your issue. Noting that you have put new discs on and new pads. Failing it being the pads. You may find that the discs on your particular bike unfortunatley hit what is known as the naturally occuring resonating frequency.
What that means - a resonant frequency is a natural frequency of vibration determined by the physical parameters of the vibrating object. In your case the combination of bolts used to mount the disc, the disc pad pressure applied, the speed of the wheel spinning, the material used to make the disc, the pad material. At certain speeds the wheel may well be spinning at just the right frequency ( number of spins) to hit this natural resonant frequency. Hence causing the discs to being to sing and squeel.
Ever see in cartoons when the fat lady sings and breaks the wine glass or a window. This is caused by the frequency of her voice hitting the resonant frequency of the glass. The glass will actually beging to vibrate harder and if applied long enough will break. Similarly this may be occuring to your bike discs where they have just the right combinations of torque, wheel spin etc to start to vibrate abnormally to cause them to squeel at certain speeds. you will probably find that at some speeds thay will not squeel. My bike for example when I had the stock factory fit pads on, at 70 mile an hour when apllying the brakes made no noise at the front disc, however different story at 25 mile an hour they would hum for about three seconds and then stop humming as the wheel slows down to 15 mile an hour.
I am assuming you have checked all setup of brake is correct. First thing is change your brand of pads, I know they are new, but not all pads are the same. The ones you have may well be made in such a way ( slightly harder than other brands for example) to be just the final item that is causing your discs to hit their resonating frequency and hence squeel.
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