Bought a home 10 yrs old - it has propane heater with electric start, gas stove & dryer. Our propane tank sits outside. We have been in the home almost 7 months - now starting to hear this knocking noise day & night - not consistent, more intermittent but a couple times a day or night. I can hear it make that noise in bathroom, then is family room, then in near the fireplace. All of these rooms are in straight line to the propane tank outside. I don't hear it in the laundry room or kitchen. I hear it when the there is no activity in these rooms. Fireplace has never been turned on. Btw: I live in Texas - extremely hot, drought, temps are now dropping to 80s. Thoughts on what is causing it? Who should I call and what do I want them to check?