Strange noise from my fireplace this morning. Sounds almost like rushing water and I can hear it throughout the house. Very similar to a toilet filling with water after a flush. I hear it for a second and it goes away, but comes back. Sometimes in a matter of 10-15 secs, other times a minute or two. It's gas, but doesn't sound like it's coming from the gas pipes, but instead from up inside the chimney. Everything is on the other side of the house too like water heater, water main, furnace, etc. I'm pretty sure there's no water pipes on this side of the house and only the gas lines. It's not loud at all in the basement either . It's also louder when I open the flue. Any idea?