I will admit expressions are my downfall when it comes to After Effects. It doesn't help that anything dealing with numbers/math are a foreign language to me.
Anyway I need to make a still image of a dangling earring rotate back and forth on an ear. I tried wiggle and other expressions but was unsuccessful. I then created the effect manually with rotation keyframes. I am sure there is an easier way to do this especially since the image will be in the FG and blurred so the movement does not need to be exact just the illusion is enough.
I will need to also start and stop the movement which I think could be applied to the right expression with a slider control and just keyframing to 0.