Your car uses the "ZF"-style power steering pump which has a history of becoming weak due to worn vanes. There are two clues to look for. The first is the power assist will come back when you raise engine speed slightly. The second is the lack of power assist affects turning in both directions equally.
When internal leakage occurs in the rack and pinion's spool valve, it almost always affects turning in just one direction first, then in the other direction a few weeks later. That loss of assist is intermittent and will go away when the power steering fluid warms up. Power assist will return at first within less than a minute, but the problem will get worse over time. Eventually there will be no power assist in either direction. For this cause, raising engine speed never helps.
Your observation of no noise also points to the pump. With worn vanes, it is not doing anything, just like if the belt driving it was broken. When you hear a buzzing noise, that is the pressure-relief valve vibrating open and closed. That is related to fluid pressure, and pressure is only built up when the pump is working. Air in the system will cause that valve to vibrate too, but again, only when the pump is building pressure.
Friday, June 16th, 2017 AT 4:42 PM