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?
Yep - this is the image stabiliser kicking in. Note if you flick the switch on the lens to turn IS off, the noise you describe will cease. (But obviously, you will then lose the benefit of it). In short - it is perfectly normal, nothing to worry about, and helps you achieve sharper shots by minimising the effects of camera shake.