What is the sound that I'm hearing from Canon EF 24-105mm F/4 L lens?

Question

I've a Canon 600D with Canon EF 25-105mm F/4 USM L lens. When I half press the shutter I can hear the US motor sound trying to focus on my subject, however I hear another sound, it's lower than the sound that the US motor makes when it's trying to obtain focus. And when I raise my finger of the shutter I can hear that the sound of the US motor has stopped while this other sound is still going few seconds and then it stops. I wonder what is this sound?

Asked by Akram Mellice

Answer

You're hearing the IS start to activate.

Answered by rfusca

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