Approximately 20 seconds after the system commands heat we hear a loud thumping
Noise coming from the 2" plastic vent inside the wall next to our bedroom. It lasts for
Approximately 45 seconds with 5 second intervals. The furnace is about 20 yrs old and
Works fine otherwise. It started last year and we had a HVAC person out who was unable to determine the cause. We have our house up for sale and don't wish to pass this problem onto the future homeowner. Plus, the noise wakes us up at night. Appreciate any help.

Is the pipe anchored? Noises can come from many sources with different causes and if the ON SITE service company cannot find it you might try another local company with better luck.

Is the noise regular. Was the noise happening when the tech was there(hard to find a problem that isn't there). If the noise is there when the tech is, they should be able to locate it.. If it happens at a certain time of day, every day, make sure to have the tech come when its happening. Good luck, and id be cureous to know what you find.