The thing was keeping me await at night. So I just paid a professional repair guy to trace it down. It was the gas meter outside the house that is doing it. It is now gone after they put in a new meter.
The sound travels through the pipe making it sound like it was coming from the furnace. Once we went outside of the house, it became obvious where it really came from.
What External Gas Value were you talking about? Thank you for your help. If you meant outside, you are better than the professional repairman. He had to almost take the furnace a part and stop all moving parts to find out.