OK - the clicking started again today. I found the relay tray mentioned above and found the relay that's making the noise (don't know why I thought it was coming from the middle of the car before). I swapped it with an identical relay next to it to see if the relay was bad but it kept making the clicking sound exactly once per minute. It's the third relay in from the back of the car. It's the relay on outside. Here's a picture of it removed.
Thanks for any suggestions on where to look next.