When using an A-mount lens with the mount adaptor, the aperture mechanism emits a sound in the following situations. This is not a malfunction.
NEX-VG10E: When shooting in [M] or [A] mode.
NEG-VG20E/NEX-VG900E: When adjusting the aperture manually using the [IRIS] button
This sound can be reduced if an external microphone with 3.5mm diameter mini plug is connected and placed away from the lens. If the sound is still annoying, you can cancel the audio recording (NEX-VG10E) or adjust the audio rec level (NEX-VG20E/NEX-VG900E).