(1) A suggestion is made that the high-pitched notes emitted by some induction motors are due to a side-pull arising from an unsymmetrical field which may be produced when certain numbers of rotor slots are used.(2) A simple case is first considered and the dissymmetry shown.(3) and (4) The forces produced are indicated and their effects considered.(5) The field is analysed and found to include pairs of components, such that in each pair the numbers of poles differ by two. The interferences between such pairs of fields produce the effects observed.(6) An elementary investigation is made which gives an expression for the frequency of the note produced.(7) Illustrative examples are considered.(8) The principle is extended.(9) A more general investigation is made.(10) A rule is developed for determining what numbers of rotor slots should be avoided.(11) Some experimental results are given.