I have a Nikon D5100 and a used Nikkor 300mm AF-S f/2.8D IF-ED II, where the two usually work great together. However, if I use autofocus a few times in a row (usually when the change in focus distance is small), I hear this funky motor noise that sounds like a motor is winding up, and my autofocus stops working until I switch to Manual and back to Autofocus mode (A or M/A). Here's a sound clip of me focusing a few times, the motor noise, me switching from Manual focus and back, and successfully focusing again:
Any idea why this could be happening? I'm hoping my autofocus isn't going out.