The brakes are making noise
Brake pads are rubbing on the rotor
Pads need to be adjusted. Adjust the cable tension to shift the left pad, use an Allen key to adjust the right pad. The pads should be adjusted so the rotor spins freely in the middle of the two pads
Brakes not bedded in properly, material buildup is causing noise
Lightly sand and clean rotors and pads. Bed in your brakes.
For a foldable bike frame folding clamp may be loose
Makes sure the frame folding clamp is secure. Tighten if necessary