Is this one of two thick black hoses that run through the outer bulkhead? They look like they have an in-line pump on them. If so I'm not sure they are to do with the power steering. Mine makes a noise like you describe but it is related to the heating (non AC UK car). It makes the same sound with just the ignition on, I think it's circulating fluid. I've noticed it stops if you put the temp wheels down to MIN.