I read a lot of reviews when buying Nikon D5100 but no where did I hear this. Check this video on youtube. When you zoom in/out you can clearly hear the lens itself. This is highly undesirable. Is it normal with any DSLR or with Nikons only? Is this a genuine flaw? Canon Guys, do you hear this zoom as well?
Noises on the camera, such as the zooming of a lens or even an IS/VR motor, are often picked up by on-camera microphones. That's the disadvantage of an on-camera microphone and isn't specific to this camera or the Nikon brand.
The solution is to use a better mic that's either mounted off-camera or is a shotgun-style mic on the hotshoe where it won't pick up camera noise.