Lennox Furance Odd Noise

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I am chasing an elusive noise from my Lennox Furance. I have had service people out but they can't recreate the sound after countless attempts. I am never one to start swaping parts to see IF this works....

So here is the description -
  1. Thermostat calls for heat
  2. Inducer blower starts up and runs as it should. There are no odd noises or vibrations and it runs for the appropriate time.
  3. Gas Valve turns on and ignition starts.
  4. Burner appear to be ingniting and buring efficiently.
  5. Almost immediately after ignition (if the elusive, sporadic, and inconsistent event occurs) a noise best described as a hum/vibration starts and it can last from 1 second to up to about 20 seconds.
  6. After the noise stops the main blower will cycle up to full speed and the furance runs normally for the cycle with no additional noise. Furnace blower motor can be in circulation mode or off - The sound can occurs in either situation in the instance I am describing it is in cirulation mode - i.e. slow blow
It is impossible to replicate it. I and the technicians have tried literally dozens of times and there is not a pattern or trigger that can be found.

The noise is annoying but does not appear to impact the furnace function. The issue does not occur in air conditioing mode.

I have finally captured the event on video after sitting at the ready with my phone for several hours. I will figure out how to post it - most like youtube and add a link.

In the meantime if anyone has any thoughts they could share it woudl be greatly appreaciated. Otherwise looks like another service call with no result other than we can try X and see what happens.

Is there any other information that I could share that would be helpful?


The wife is still training me.
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Orlando, Florida
Fitter30 is suggestion is the first item to look into. Also check the air mixture shutters for spider webs and dust accumulation. Since the noise starts with the gas flow, possibly it's the gas valve not fully opening right away or if there is a gas regulator the diaphragm inside of it is vibrating.
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