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I have an air duct in my basement. It leads from my furnace (I think?) to the outside world.

However, as you can see from this video, it is "twitching". Every second or so it jerks slightly and makes a noise.

https://youtu.be/u-WKaxWBnw4

The portion of the duct shown in this video is the end of the duct that terminates outside the house. The rightmost portion of this duct leads to an opening on the exterior wall of the house.

When the furnace switches on, the twitching stops. What's going on? Do I have an alien living in my air-duct?

Update: The end of the duct is not connected to the furnace. It is connected to a clothes dryer. But I still don't know why it is clicking like this.

  • Sounds like something switching back and forth. What is on both ends of the duct or in the line somewhere? – HazardousGlitch Dec 23 '18 at 12:52
  • Update: The end of the duct is not connected to the furnace. It is connected to a clothes dryer. But I still don't know why it is clicking like this. – Saqib Ali Dec 27 '18 at 23:59
  • My answer still pertains (although I changed "furnace" to "dryer"). Go find the flapper, and it'll be clicking. Hold it either open or shut, and the clicking (and the duct throbbing) will stop. – Daniel Griscom Dec 28 '18 at 1:24
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The outside end of the duct has a flapper to prevent air (and critters) from coming into the house when the dryer is off. However, even with the dryer off, there can still be a small amount of airflow through the duct, e.g. from inside/outside air pressure differences. It seems that in your case there isn't enough airflow to keep the flapper open, so it swings closed, making a "click". Then, as the (small amount of) pressure builds up, the flapper pops open to let a burst of air through. The changing air pressure causes the flexible duct to throb. Repeat twice a second, and bingo: alien living in your vent duct.

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Is this air duct part of a fresh air intake? Is it connected to an active damper which controls how much air flows through it? If so, the damper may be switching on and off when the HVAC circulating fan is off.

Alternatively, wind blowing across the vent on the exterior may be setting up oscillations when the HVAC fan is off.

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