I experienced a similar noise on my 126. You might have air in the ps system. My 2 cents.
You may wish to bleed the system. Some have recommended pulling the return line and pouring fresh fluid in from the top while letting the old drain out the return line. I'm a little snake bit there as I played hell once getting that line reconnected. Others may not have experienced that problem.
You could simply siphon out the top with a turkey baster, replace the ps filer, then refill. Some say to turn the steering wheel from side to side to bleed. One of my asian vehicle shop manuals says to elevate the front a bit so as to get the front wheels off the ground. You just need enough so that the wheels may be turned.
I used this approach on the MB last time I changed psfluid/filter and the groaning seemed to go away.