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I have a new, 3-year old, high-efficiency furnace in my home. Last year it started making a slight whining noise that was audible during the very first stage, before the blower motor started. No problems. Some weeks ago I started to hear a rumbling noise when the furnace kicks on--again, first stage, it's not the blower. The rumbling sounds a bit like the noise my fridge fan makes when its a little off balance. What might be causing this?

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Gas furnace, forced air. –  David Jan 15 '13 at 3:56
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3 Answers 3

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I'd have a look at your draft inducer motor. Do a Google image search for "Furnace draft inducer" to see what it looks like. I has the same kind of impeller as your main blower but on a smaller scale. You may have some debris in there.

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I will check that out. Any advice for how to clear the debris? –  David Jan 15 '13 at 13:51
    
As it turns out, there was no debris. I cut the power to the furnace, removed the panel, and removed the draft inducer. It is responsible for the whine. I don't know if it can/should be oiled or if the bearing is failing slowly. The rumbling, however, was coming from the exhaust end of the draft inducer where there was a loose bracket. –  David Jan 15 '13 at 20:55
    
Side question: Should I lubricate / grease my draft inducer motor? –  David Jan 15 '13 at 21:06
    
Probably not. Most motors are typically sealed and require no maintenance. And the last thing you want in your exhaust gases is grease or oil. Replacing the entire motor assembly is probably not too expensive. –  longneck Jan 22 '13 at 13:22
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I have the same situation with a relatively new Lennox furnace (this came with the house, which was built new in 2008). The furnace ran OK for all of two years, and now: every time the thermostat calls for heat, there's this rumbling / vibrating noise. I've isolated it to the cowl that covers the draft inducer motor.

In my case, the only option is to put in a new inducer motor - your mileage may vary.

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I'm assuming since you mentioned a blower motor that you have forced hot air. Your system has two thermostats -- the one you use to set the room temperature controls the burner, and a separate thermostat internal to the furnace controls the blower fan, switching it on when the burner has warmed things up enough to not make a cold draft, and back off when things have cooled down after the burner has been shut off.

You are probably hearing the burner -- if oil, there is a motor, which may have a small vibration; if gas, you could be hearing the combustion air being sucked in, or possibly the solenoid that controls the gas valve.

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