Most likely the power transfer unit, part of the hydraulic system. It transfers hydraulic power between the two main hydraulic systems when it senses a difference in the two systems. You'll often hear it during taxi in an out if a one engine taxi in or out is performed as only one engine driven pump is running this that hydraulic system is being pressurised (green). The PTU takes some of that power and pressurise the yellow system. It may be heard at other times but it is most common at taxi.