Is that the hooks dropping down into the latch mechanism?
Mine has been "clicking" when I close the screen for a couple months...
It started where I had to close the screen slowly/a couple of times for it to do it. Now it does it all the time.
I have not really worried about it though...
The MacBook Pro, and normal MacBook do not have latches, well the newer versions anyway. They close magnetically. So if I'm correct you are hearing the magnets. It's nothing to worry about.
The MacBook Pro does indeed have latches. They are two little hooks that drop down magnetically from the lid and drop into two holes at the very edge of the palmrest. They do make a barely audible click when dropping, although the OP doesn't believe this is related because it occurs in a position where the hooks are nowhere close to having dropped.