The n/rev roughness/vibration below 30 kts ( and in my experience* ) in a descent at more like
800-1300 fpm, the phenomena you refer to isn’t stall, but the advancing blade going thru the tip vortex created by the preceding blade.
The vibration/roughness can be attention getting, if one is successful in doing this in a steady state manner ( need non-gusty wind conditions ).
*Typical flight load survey test plans at Sikorsky would include a so-called “ rough approach” for each condition ( weight/center of gravity ). The vibratory structural loads in some areas, in some machines can be high enough to influence some structural lives. In my experience the CH-54 was particularly “interesting” when put into this condition.
BTW-nothing sacrosanct re the rate of descent I quoted for the Sikorsky machines-it will depend on the specific rotor and weight. We would typically do the flight loads survey work at design and max or alternate gross weight.
Chr: title of your post included stall in a dynamic turn, but your post did not include a turn but a straight descent, and my response was based upon that. Did you intend to discuss stall in turns?