I was thinking more along the line of the tremors or nodding being in tune with mechanical vibration in their environment. Though the possibility of it being part of the cause and effect is interesting to consider. Of course that's probably what you answered. Just to clarify because I may have bungled the question-
Let's say someone with Parkinson's is stressed or anxious and begins to nod or tremble. Has anyone studied the visible physiological movement in relation to, let's say, the sound and vibration produced by a refrigeration unit, or similar object, in their immediate environment?