I've observed this carefully for the last year and it's only in the shift going from 2 to 3, although a downshift from 4 to 3 might make too much noise to hear it. In any case, if I deftly back off the accelerator just before it makes the shift, the noise is abated or avoided altogether. But even under perfectly modest acceleration it will do it just about as noticeably as when under heavy acceleration.

Any ideas?