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Mark Henley
Mark Henley, Journeyman Technician
Category: HVAC
Satisfied Customers: 3217
Experience:  UA Journeyman Pipefitter , HVAC, Refrigeration, DDC controls. 26 years.Commercial & residential
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Why is my furnace making a knocking noise?

Customer Question

My furnace is making a knocking noise. It does not happen all the time, but it is happening more regularly. There is also a slight gas odor sometimes. The noise appears to be from the flames pulsing after they are initially lit. My gas valve is the WHITE-RODGERS 36g24 type 205 GAS VALVE. I have include 2 images of the furnace and a short video of the issue happening here... Is this dangerous? What can be causing this? How much should it cost to have it fixed? Thank you.

Submitted: 7 years ago.
Category: HVAC
Expert:  Mark Henley replied 7 years ago.

Thanks for the visuals,

Can you put your hand on the gas valve while this is doing it and tell me if you can feel the valve pulsing in tune with the flames ?

Let me know

Customer: replied 7 years ago.
Ok - I did. And - yes, it does pulsate with the flames.
Expert:  Mark Henley replied 7 years ago.

You really need to have the gas pressure checked before the appliance and while its running but its a pretty good bet that this valve has water in it or some other type of debris. I see you do have a drip leg at the furnace in the gas line I would take that apart and see if you get anything out of there as far as crud or if you can see water trails. I think you are going to have to start with replacing the gas valve if you don't have any means of checking the pressures.

The gas smell is probably from the valve also.

To have the valve replaced will probably cost you about 300 dollars.

Customer: replied 7 years ago.
Yeah, it happened the beginning of last year and then the last 2 months of winter it stopped completely.

I was hoping it would do the same this year. Could it be an obstruction in the exhaust pipe or something?
Expert:  Mark Henley replied 7 years ago.

Shouldn't be, the pressure switch should detect a blockage and kill the gas valve if the vent was blocked, that's why I figure the pressure isn't changing much in the exchanger and whatever it is would have to on the incoming side of the exchanger.