I have a 2-story colonial house (with a basement) in Michigan. I have forced-air heating. The house was built in 2001. On the upper floor, my daughter noticed that a clicking sound (similar to an old light-switch) started which sounds like it's coming from within the wall. The wall is between two bedrooms. It is made of drywall. The heating register for her bedroom is on that wall, but is about 5-6 feet away from where the clicking sound seems to be coming. We timed the clicks. They are occurring every 23 seconds, like clockwork. Lastly, they seem to be coming from the upper area of the wall. I have not yet looked into the rafters of the house. I will do that as soon as I can. I can barely hear the click when I stand in the living room below the bedrooms. It's definitely emanating from the upper floor area.
Any ideas as to what could be causing this? I'm the original owner of the house and this is a new thing.
UPDATE: As I was looking for appropriate tags for this question, the clicking started coming pretty fast for about a minute or so. Now it's back down to a regular interval of about 10-15 seconds. Also, I looked in the rafters and was not able to see much beyond a lot of blown-in insulation.