A likely cause is the Sigma lens’s HSM/ultrasonic autofocus motor. These motors can produce a faint high-pitched whine that some people can hear, even though they’re marketed as “ultrasonic.” If the sound only occurs during focusing and the lens otherwise works normally, it may be harmless.

However, if the ringing starts as soon as the camera powers on, happens constantly, or seems unusual compared with how the lens used to behave, it could indicate a fault or poor contact rather than normal motor noise. One community report described the noise going away after cleaning lens contacts/internal connections, but opening the lens is risky and not a first step.

Best advice: if the sound is new, persistent, or the lens has focusing/stabilization issues, have the lens checked by Sigma or a repair shop. If performance is normal and the sound only appears briefly during focus operation, it may simply be the AF motor noise.