I haven't used the chip, so I don't know what the chopper circuit sounds like, but that would be my first guess. In short, its a feature, not a bug.
Motors, and other inductors typically have maximum current limits. At some point electromagnets saturate(and get inefficient) and wires overheat(melt/short together), and in the case of motors, magnets get demagnetized(and it stops being a motor). For various performance reasons, steppers usually limit the current going into the motor - that's the purpose of the SENSE1, SENSE2 lines going between the two chips. The chip basically chops the current off when it hits a given threshold, and then turns it back on a bit later - hence the buzzing.
The reason why it does this is to hold the position of the motor so that it doesn't slip. If you want to disable this feature, use the enable line of the L297 chip to stop it when you want it to "let go"