You say it's a groan. Not a clunk sound. I had a 1985 300SD with similar problem. If you look into the front wheel weld up to the left you can see the torsion bar with a rubber bushing around it. What I did was to spray a liberal amount of WD40 into the area where the bar and the rubber bushing make contact. I did this on both driver and passneger side. Lifting the hood if you look below the wind shield and behind the engine you can follow the path of the torsion bar across the width of the car. I sprayed anywhere I thought the bar made contact with the car.
At least for me the sound stopped. Sometimes but not often I have to repeat this procedure.
If your sound is a clunk, someone in this forum suggested last night on another question the problem may be worn guide rod mounts.
