After switching off my kitchen lights, there is a somewhat loud "click" almost exactly 7 minutes later. Infinitely reproducible, with no other discernible symptoms, and it seems to be otherwise harmless. The house is new construction, only about a year old, and the sound has been happening from the start. I originally thought it might have been coming from one of the GFCI outlets, but the outlet works fine and after listening to it a hundred times I now think the sound is actually in the wall somewhat above the outlet.
I have found a few posts online (e.g. from a DIY forum) but the advice seems to always be, "replace the receptacle" or "don't worry about it." I am not worried about the sound, and honestly don't really even care to stop it, I just want to know (because of naturally scientific curiosity) what causes it.