Leak at Vent

• Central air conditioning ductwork can leak where it attaches to the vent, or register, in your ceiling. When this happens, the duct should be removed and reattached to the register, ensuring it is firmly seated on the metal flange of the register. A large clamp is usually used to secure the duct to the register, and aluminum or duct tape can be used to seal the area. Run your hands around the flange and register once it is reseated and taped to see if you feel any leaking air, and tape any gaps.
  
  

Duct Leak Inline

• If your central air conditioning duct is leaking and making noise in-line, meaning not at a register or at the central air unit itself, the duct may have to be replaced. You can temporarily fix a leaking duct by taping it with duct tape, but the leak was likely caused by the plastic of the duct becoming brittle and splitting. More holes will open, and more leaks will emerge. Replace the duct work now to save you another trip to the attic later.
  
  

Leak at Central Air Unit

• Ducts can also leak where they attach to the central air unit itself, which should be treated in much the same way a leak at the register is treated. Remove the duct entirely and reattach it to the unit, ensuring it is seated properly. Secure the duct with a clamp, and tape the area until you do not feel any leaks emerging from the site.
  
  

Identifying Sounds

• Leaking ducts have identifiable sounds that can help you isolate where the leak is coming from. A flapping noise indicates a large split, with a piece of the plastic duct moving as the cold air leaks into the attic. A small split will have a buzz-like noise, and a pinhole may have a whistling or high-pitched squeal as air escapes. Use these sounds to track down your leak, or leaks, and seal them up.