Got my D7100 with the kit lens about a month ago. then bought the AF-S DX 35mm f/1.8G about 3 weeks ago and been using this lens ever since. Until yesterday, all of a sudden the lens is making some weird noise during auto focus (auto focus still work but making a louder noise then usual). I turn the camera off and turn it back on, then the auto focus not working at all. then i tried taking out the battery and reinstall it, removed the lens, remount it again and all didn't work. finally i mount the kit lens back and auto focus works again. then i process to remount my 35mm lens and its back to normal. Is that something i need to look into or not worry about? anyone got the same happened to them?
Since the kit lens autofocuses without issue, it is probable that the noise comes from the 35mm lens as the G series lenses use electronic control for their autofocus. AF-S lenses do not use the mechanical screw drive for autofocus.
Personally, I would either have the lens exchanged or repaired under warranty. It will never heal itself and Murphys Law says that it will die at the worst possible time.
It's possible that you hadn't mounted the lens securely on the body, and the electronic contacts weren't making good contact. I have done this myself on a few occasions resulting in exactly those symptoms.