Don't know if this will help or not, but I have a Nikkor 70-210 and a nikkor 300 that have been doing the same thing for years now. It's really bad when you manually focus either of them fast, from one extreme to the other. Other than the noise (which has actually helped out on a few occasions - deer tend to look up to see what's making the noise, but it's not so loud as to scare them away), both lenses work fine, having suffered no difference in speed or optical quality - so I've been living with it. On my lenses, it sounds almost like the AF coupling on the lens is resonating (sp?) within it's hole. I am sure of one thing though - DO NOT try to oil it, unless you want yuck and gunk inside your lens.