I have a furnace (Natural Gas) that aove r the laswt three years that rumbles when the heat goes on. At first I thought it was our neighbour taking his garbage out over a newlay laid concrete stairway. We know it is not that . He doesn't have that much garbage. It is beginning to concern me but not my husband. Can anyone give me an idea of what it might be. Worried and thanks for any help.

You could have a duct that is no longer tied down securely or there may be a restriction in your duct work. How comfortable is the house. Slow to heat up?

Usually as mentioned by ballengerb1 the duct work can be the problem but for your safety and to elevate any safety concerns have your unit inspected by a local HVAC contractor of your choice. Piece of mind is a good thing.