#1. "RE: sensor cleaning" In response to Reply # 0
Welcome to Nikonians! What camera bodies have you used in the past? I have never felt my D300, D700, or D3s vibrate when I cleaned the sensor. I will bet that your D3100 is working normally. Good Luck and Enjoy your Nikons!
#2. "RE: sensor cleaning" In response to Reply # 1
Previous camera was Olympus e510. Other cameras are canon f1 and a vintage speed graphic. I still use film cameras and have a fully functional dark room. My father introduced me to photography fifty years ago. I still have all his large format cameras and use them often. My nineteen year old just received a Ain his photography elective and tells me if I ever say rules of thirds again he'll scream