Check the pulley alignment and see if that is where the noise is coming from. If it is at low rpms and goes away then I would look there first.
If it is in the steering box then it might be pressure related. Higher pressure makes it go away. Otherwise it would get louder as rpms increase.
Do the old hose in the ear trick to help find the source of the noise.
72 Series III 109 SW Diesel, 03 RR, 98 Porsche 986, 01 Porsche 986 3.6 L