That noise you hear is the hydraulic PTU (power transfer unit). It is not a primary source hydraulic pump. It functions basically when there is ( I think...Since I am Boeing now...and brain-dumped bus) approximately 500 psi difference btw the green and yellow hyd systems. It transfers power from one hyd system to the other system to boost the lagging side pressure. Anytime one engine is shutdown and the other is operating, you will hear the PTU operate (unless the yellow pump is electrically operated or the PTU shut off). Hence the reason you hear it during one engine taxi or shutdown. It sounds like somebody sawing on the aircraft hull.