When i close my mouth so my teeth are almost touching, my teeth start slightly chattering. what could cause this?

Tired jaw: May be you have a condition where muscle control is weak. I would consult dentist who deals with myofascial disorders. Good luck.

Muscle spasms: The chatter you have is due to muscle tremor due to the muscular exertion to maintain the jaw in a unnatural position. The longer the effort the stronger the tremor and chatter. This is known as benign fasciculation syndrome, fasciculation meaning muscle twitching.