First locate the origin of noise. Have someone turn on/off the fan or blower switch while someone watches and listens to locate the noisy duct(s). (Hopefully the ducts are easily accessible). A duct may need to be reinforced due to improper duct construction, installation, duct size, metal guage, or lack of proper reinforcement technics used such as cross breaks, 90 deg. bends, stifferners, etc. After you locate the affected duct(s) you will need to determine what type of reinforcement or bracing that will be needed to stop the movement (expansion and contraction) of the duct. There are a variety of ways to do this and one way is to attach angle iron or other suitable material (sized accordingly to the strength needed) across the duct reaching both ends over the center of the weakest point or the affected area of the duct. When using sheet metal screws to attach angle iron you must be carefull not to strip the self tapping threads.