That appears to be the pump for an automatic load-leveling system. The arrow points to the link between the rear suspension member and the height sensor on the body. The wires you might see going toward the lights is just because the various circuits have wires in the same harnesses.
If that pump is making noise for different lengths of time as you mentioned, it would be because the back end needs to be pumped up different amounts depending on the size of a leak or the size difference of the weight in the car after you load or unload something.
Thursday, December 6th, 2012 AT 2:10 AM