The lens vibrates when the shutter button is pressed halfway or the AF-ON button is pressed with Pre-Capture set to On in AF-C focus mode.

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The lens constantly focuses before each shot to enable precise AF in continuous shooting, including when pre-capture is used, which may be felt as vibration or sound.
This is not a problem, because the operation is the same as in regular continuous shooting without pre-capture​​​​​​.