When current flows thru a coil of wire, an inductor, or a motor winding, etc. it creates a magnetic field around the wire and thru the center of the coil. Each coil, loop or turn in the winding has it's own magnetic field, and when the current changes direction the magnetic field reverses also. These reversing fields will interact with each other to cause the wires to jump and move about due to the magnetic forces, and the movement of the wires will produce sounds and squealing noises.
The motor manufacturers wrap and tie the windings with string and coat everything with varnish in an attempt to limit the vibrating motion of the wires and reduce the noises. No two motors are wound, tied or dipped exactly the same, so the sounds can vary from one to an other.
my 2¢, kiev kenny
kiev = kenny's innovative electric vehicle