If the pads touch the rotor only sometimes.
You may have the rotor that is not perfectly flat.
It occurs, even on new ones.
If you are in this case. Rotate the wheel slowly.
Remember the spoke, in front of the brake pads, where the noise starts.
Rotate until the noise stops, and look at the spoke too.
Take the rotor at the middle between the two spokes, with your hand.
And try to push(very gently) the rotor inside.
If it gets better, but still hearing a noise, push a bit harder.
If it get worse, pull the rotor outside instead.
After a few trials, you should get the rotor flat enough to avoid any noise.
I had this on my build with Hope's rotors.
