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Old 01-24-2022, 12:34 PM   #1
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Peculiar Furnace Noise

I have a Fleetwood Flair with propane furnace.

The furnace functions OK but while the burner is lit, makes a vibrating sound which is quite annoying. It is relatively new and has always done this (since new).

It's not the fan or the fan motor.

The noise is only present with the burner lit. Here's the sequence:

Furnace fan goes on from thermostat
Fan runs for a bit
Burner lights
No unusual noise for several seconds
As burner heats up, vibration/resonating noise ramps up
Burner shuts off when temperature reached
Unusual noise immediately ceases when burner goes out
Fan continues to run as normal for a few moments

Something is reacting to the burner operation, especially once it warms up to a certain temperature.

Ideas? Anyone else run into this?

The sound is a little like the sound you hear from cars with the bass turned way up, low frequency I mean. Resonating.
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Old 01-24-2022, 12:44 PM   #2
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That rumble is usually associated with the burner flame flow. You can check for any obstructions (inlet/outlet) and the nozzle itself. Or it could be the flow rate of the gas. But I think the burner should come out, not hard to do. Look in your manual for trouble shooting ideas.

O-B will have to chime in to hopefully give you a better diagnosis.
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Old 01-24-2022, 04:20 PM   #3
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Thanks. I just now got down close to it and put my hands on the sheet metal case while it's running. The noise seems to be coming from the sheet metal, either the outside box or possibly something inside, or a combination. I can affect the noise by pushing on the metal.

So the 'normal' rumbling caused by the burner is apparently transferring to the sheet metal and creating the 'not normal' reverberation of the sheet metal.

I'm not sure if this traces back to the burner flow issue mentioned upthread, i.e., there is something wrong with the flow which is creating more rumble than usual? Or is there something wrong with the metal shrouding someplace?

The 'normal' rumbling I can put up with. The reverb not so much.


Dometic model DFMD30141
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Old 01-25-2022, 06:48 AM   #4
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A lean burn can cause the reverberation that will vibrate anything loose or flimsy.
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Old 01-25-2022, 06:58 AM   #5
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A lean burn can cause the reverberation that will vibrate anything loose or flimsy.
Ditto. But be careful. Starve it for to too much more and she'll begin to "pant". One of my engineers destroyed the breech on one of my B&W D-type boilers.
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Old 01-25-2022, 06:14 PM   #6
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Forgot solutions . Check LP pressure for regulator, they can get oil-logged. Clean or replace burner nozzle. Don't poke out with anything.
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Old 01-25-2022, 07:00 PM   #7
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Correction to my earlier post which said I can affect the noise by pushing on the sheet metal -- I cannot.


I have posted a video which illustrates this issue, link below. Noise starts at about 1:30 in the video:


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Old 01-25-2022, 08:30 PM   #8
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Not the "rumble" I expected it to be... it is a higher frequency. You say it only is present when the burner is in operation?
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Old 01-26-2022, 02:01 AM   #9
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Still sounds like lean burn the burner. It's fan moves a lot air through small exhaust, it's not a small induction fan. Our huge inert gas generator/boiler will resonate when way lean . It will crack 1/2" studs on the second pass doors if goes on long enough. Hopefully resident expert OldBiscuit will chime in.
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Old 01-26-2022, 05:47 AM   #10
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Give us a listen outside at the exhaust port. If the flame is starving for air it may begin to purr like a cat or a hovering hummingbird. Good luck!
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Old 01-27-2022, 08:43 AM   #11
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Not the "rumble" I expected it to be... it is a higher frequency. You say it only is present when the burner is in operation?
Yes. In the video, at 0:13 the burner lights. No noise until 1:30, after the burner heats up.

The video was shot when the furnace was cold. After the furnace has gone through several cycles, it doesn't take as long for the noise to occur after the burner lights (since it's already warm).

Also the noise stops immediately when the flame goes out.

I read the service manual and there is no adjustment for the fuel burn on this model. And it has done this since new, five months ago.
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Old 01-27-2022, 09:18 PM   #12
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The Lp orifice may have speck of something in it from install, bit of tape, old pipe sealer or plain old rust / dirt. Can you block off burner air inlet "some, for a moment" with your hand see if it quiets down. If you just changed/installed furnace was there an issue with the first like low LP pressure may cause. Biggest chance of dirt in orifice is when first installed.
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Old 01-28-2022, 11:10 AM   #13
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you guys are right about the orifice, or sometimes the lean burn. sometimes, its really hard to get an owner to understand why you have to take a couple hours fixing this.
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Old 01-28-2022, 11:37 AM   #14
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We experienced a similar problem with our furnace which ended up being na loose knock out panel in the furnace casing, rattling after the furnace warmed up. We placed a shim between the furnace and the surrounding framing which eliminated the annoying noise.
This may not be the solution to your problem but it was ours.
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