What is causing clicking sound after light switches off?
Answering this myself since I have an answer and this has gotten quite a few views.

The answer is that the sound was thermal, but not anything dangerous. After much experimentation, I discovered that one of the cans warps ever so slightly when it gets hot and after a period of about 7 minutes this cools enough to "pop" back down as it contracts. It also doesn't happen until it hits a heat threshold that occurs after about 10 minutes. Less than 10 minutes of on time yields no pop.