Vibrating spring like shutter sound in Nikon D3100? - 1

I recently bought a Nikon D3100 online(Box piece). The camera seemed great at first with its smooth shutter sound. Around 1200 shutter releases done. I was taking some shots with 30 second exposure and in bulb mode when i started to hear vibrating spring like sound at the end of shutter release. Now i can hear the sound even at shutter speeds of 1/320 and more (Maybe because i'm paying more attention to the sound now). The spring like sound is annoying me.gives a cheap feel to the camera. And i'm also worried about the shutter life now. What do i do?

Since the camera is new I'd contact Nikon

Call Nikon 800. NIKON-UX

That has never happened on any of the Nikon dSLR cameras I have owned (since 2002).

All I can think of that may make such a sound is if you had the lens in the VR mode, mounted on a -tripod and the auto-focus mode set to continuous. That would make the lens auto-focus motor and VR system stay activated, trying to refocus and correct for any camera movement (possibly caused by the mirror flapping up at the beginning of the exposure).