One of mine seems to chirp 4 times (20 second intervals) in the middle of the night. I have used compressed air to blow it out but it did it again at 3am. It only chirps 4 times and none of the other ones make a peep. Unfortunately, 4 times is more than enough to wake me and get the adrenaline pumping. The interval is very consistent. Whats the deal? Any ideas?