Water Heater Flue Pipe Making Knocking Sounds

[Azimuth]

Hi Everyone,

We are in a 2 year old house. Every time the water heater turns on we hear loud knockings sound in the attic. We were able to find the pipe causing it - It's the flue pipe coming out of the water heater on the 1st floor and going upto the attic and having the exhaust out. The pipe is definitely expanding or cooling which is fine but it makes loud banging sounds every time it expands or cool. Surprisingly it's not happening in any of the near by homes built by the same builder/plumbing company.

Any idea what could be causing this and how to mitigate the loud banging noise - It's loud enough to wake us up in the middle of the night. Attaching 2 videos: First one is from our bedroom where the loud knocking sounds can be heard and the second one is from the attic showing the flue pipe creaking - Can feel the vibration on the flue pipe when we touch it.

Any help would be greatly appreciated. Been working with the original plumbing company for almost 2 years now and they haven't been able to figure it our or give us an explanation for why it only happens in our house and not in any of the other neighbors.

Thank you!

 

[havasu]

How about take some pictures of the flu pipe as it transitions between floors. I suspect it may be heated metal expanding on a too tightly cut piece of wood.

 

[Twowaxhack]

It’s probably in a bind. If you put a 36” piece of pipe between two fixed points and then heat it to flue temps it will grow. Or if it’s strapped tight to framing or other metal.

If it doesn’t have room to expand it’s going to make noise.

The exact noise I hear on your video