I cannot stop a loud banging sound coming from my retrun air supply when the fan turns on or when the fan stops. It does not matter if the hear or air conditioning is on. The bang takes place on the start and stop to the fan. I have isolated where it take place. I just cannot stop the sound. The return supply line in the basement is the culpret. I can recreate the sound by pressing on the sheet metal and releasing it. I can also feel the movement when the fan turns on and off. The supply line is exposed at this time so I do have some access to it. Can you please help. The bang is very nerve racking to all of us.