The very day after a hack dentist prepped an amalgam filled molar in my mouth for a crown, I not only had an extremely stiff neck, but an extremely high pitched (loud, strong, constant no matter where I went or what I did) ringing in my ear on the same side, which lasted in that extreme intensity for about a week to 10 days. This was the beginning of my very intense downhill slide from mercury poisoning, and the intense ringing, along with the very stiff neck 16-20 hours after the dental work were the very first symptoms. Followed within days by numb biceps, then forearms, then hands, followed by numerous other symptoms i've told you all about previously.

It subsided pretty much on it's own after about 7-10 days, I guess as the mercury became less locally located and spread throughout my body and even was somewhat naturally eliminated, to a faint constant ringing. It subsided almost altogether after several weeks of heavy duty chelation. By now it is almost gone completely, I would say gone completely but I still get faint reminders of it now and then.

Just my experience, though there certainly could be other causes.