I have a gas fireplace. When it gets really cold (even when the fireplace is off and pilot is out), at night I will get a "dong" noise about every 10-15 minutes. It wakes me up at night and is very annoying. I don't know what's causing this, but I'm assuming it's from the metal in the flue contracting and expanding? My other fireplaces don't do this. It's just the one in my bedroom. Any idea how to fix this?