I fortunately don't have that side effect, but it does sound like Lhermitte's Syndrome (Lhermitte's Sign)to me. I'm sure there is more info on here, just can't find it at the moment. Try to search for it. I think it's spelled correctly. I know others who have it and they described it just like you did....like plucking guitar strings (sort of a vibration) when they put their chins down towards their chests. Hope you find some relief.