Quick question - I noticed last night that my lens (stated above) has some weird motor noise going on while autofocusing. It almost sounds like the motor is straining to focus at the extremes (meaning 200mm focal length with a very close subject). It still seems to focus quickly and with normal noise on less extreme examples - Subject further away, not at 200mm, etc. Is this normal? Should I be worried about my autofocus motor going out on me? Thanks!