whining noise from front of engine
 

as my 91 560sel was warming up this morning a soft whining noise started. I pressed on the gas and it went away. now whenever i am stopped somewhere the noise is there. as i pull away it stops but i noticed on the highway that it comes back when cruising but when when you hit the gas it goes. After i returned home i looked under the hood and the noise appears to be coming from the power steering pump. I wrote about this earlier but the noise went away so how can you diagnos something that is not there. any suggestions?