We have a 2-storey house with FAG heating system. During cold whether (sub-zero temprature outside) the duct in our 2nd floor bedroom makes an intermittent thumping noise when the furnace fan goes on. It sounds as though someone is hitting the duct wall with a mallet. From the basement, the duct comes straight up to the bedroom and the goes sideways to the adjacent wall where the register is. I have been able to trace the noise along this track but closer to the entry point of the duct into the bedroom. What possible action can I take to eliminate this problem? If I can't fix it myself, then what type of professionals can I call on to take care of this?