Once it has iced, forced defrost will do nothing because the heat cannot reach that far. Since you confirm that it's the top of the evaporator tube with ice buildup (like all of us), I'm confused about your sensor comment. The sensor needs to move a couple of inches to the left from the inlet copper tube to the outlet copper tube .
Our Samsung side by side refrigerator started making noises like a fan blade hitting something. It would stop when we opened the doors. Then we saw ice icoming thru the oblong holes in the interior back of the refrigerator. What the heck. It is five years old.
One thing that helped us and I wanted to share was to control the amount of humidity in the house. if you have high humidity in the house just opening the door will cause some frost to accumulate.
This is what ours did and it was the fan motor getting ice on it. The sound was the fan hitting ice. Unfortunately all they could tell us to do was unplug the fridge and let the fan thaw and then plug it back in. It would work for about 2 days then we would have to do it again. Then we found there was a way to defrost it from the panel. We did this and now the refrigerator part is not working at all. Freezer still cools though. Ours started the noise within the first year. If you’re still in warranty, you better call for service now or they will not cover it.