tracking down harmonic resonance mystery

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we have forced air heat, when the heat runs there is a harmonic whine that resonates through a few parts of the house (which isn't very large at all anyways). i am lucky in that i'm in the offside room which doesn't hear that noise, but Ma is going to keep gnawing on me to fix it for the next six months if i don't figure it out.
when i crawl down below to find the noise it isn't there at all. i've checked all vents, nothing changed with those being in or out.
i thought of sticking a rag on the end of a hose and putting it down each vent a bit and hoping i can narrow it down to a specific run of duct work.
any other ideas to figure this out?
thanks,
songbird
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songbird wrote:

You did not even mention what kinda blower motor is running. Is it AC or DC motor? VS speed, etc.? Was this problem just popped up lately?
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On 10/23/2014 12:01 PM, songbird wrote:

What kind of dwelling? How many rooms? Has any thing changed lately? Do you have dampers? Do you have central AC?
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On 10/23/2014 12:01 PM, songbird wrote:

flue pipe going up through a chase behind a living room wall. When the gas would 1st turn on, it started making a noise like an airplane and then, once the heat exchange warmed up, it would go away. I had the original installer back several time, however, it would never make the noise when they were there. It seemed to happen only after a long cool down. I finally recorded the noise for them to hear. The 1st guy to hear the recording said, we'll replace parts until the sound goes away. His 1st guess was to replace the entire set of burners. Problem solved! Never happened again for the rest of the furnace's life.
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songbird wrote:

If the resonance goes away when you go down to investigate, be sure to close the door or whatever after you go down. Resonance in sheetmetal ductwork is affected by air pressure on both sides, and opening a door to space that is normally sealed can affect it. Once you find the panel that is resonating, you can just screw a piece of wood to it with a couple sheet metal screws to change the resonant frequency and eliminate the issue.
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Tony Hwang wrote:

i would guess AC as it is a part of the furnace, but it isn't easy to get to (it is also under the house).

we didn't notice it this summer when the air condition ran, but perhaps it was there but just not as noticeable now that we've had things off for a while.
i just turned the air conditioner on to see if that made any difference and the noise is still there aka no change.
songbird
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Stormin Mormon wrote: ...

electricity to run the blower from the power company. propane is burned for heat.

http://www.anthive.com/flowers/Way_Up_2013.jpg
1 story hexagon shaped house for the main part. each facet supported by a beam which runs up to the central fire place which is field stone. the foundation is cinderblocks around the perimeter with a central line running down the middle which also goes under the fireplace.
duct work under the house, none in the walls.

1 small enclosed bathroom next to the fireplace and another small walk through master bath next to that but the doors on both sides of that are usually open. the master bedroom is open above the other two bathrooms so that airspace connects to the rest of the main area (living room, kitchen). my room is off the north facet.

i've done a lot of sealing up around the exterior foundation but that is external.

yes, all have been removed and checked while the furnace was running, no change in noise.

yes, ran it to see if that made any difference, nope, noise still there.
songbird
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down, thinking it was electric related, I eventually found that it was the gas meter and it was transmitting the sound through the gas pipes only when the furnace was running. The gas company changed the meter and all is quiet.
You have propane, no meter, but check out the regulator that reduces the pressure, sometimes they will make a hum or whine sound and it may need replacing.
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Art Todesco wrote: ...

this is a high pitched whine and it is consistently there (with some variation which resonates).
the burners have been replaced and the blower was replaced within the past few years. this noise is not coming from the area where the heater is at, it is a high pitched resonant vibrational noise, like if you rubbed your finger over the edge of a glass like they do to make music.
songbird
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Pete C. wrote: ...

ah, well, i'll have to check that tomorrow. it's not easy to get under there for me now.

once i find it... :) thanks.
songbird
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What happens if you turn on the blower via the manual blower switch?
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snipped-for-privacy@sbcglobal.net wrote:

um, manual blower switch?
like if i turn on the fan at the thermostat control?
i would guess it will come on like any other time the furnace or air conditioning turns on.
if you mean instead, like, i have to be down in the crawl space and find some switch on the furnace, well i don't mess with that yet. i suspect that there will be no change.
it's not easy for me to get down there so line up things to do to check and after a few more days of suggestions and me trying other things, if i haven't gotten it solved by then, then i'll head down with the list and see what happens. as i cannot hear the whine down there it would involve some type of cooperation and signalling from Ma if any change happens and she isn't always as patient about such things... it may be fun, or a challenge or a distaster. we'll see... :)
was happy to get the front door weather strip replacement project done today. gardening tomorrow and maybe testing some ducts with hose and rag to see what happens.
songbird
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I have a story about a high pitched whine.
I heard it about 3:00 AM when everything was quiet. It wasn't a smoke dete ctor's beep, but could have been one malfunctioning and whining very quietl y, or something similar.
I searched the whole house. The whine was everywhere, but impossible to lo cate. But it was much louder near the ceilings, which were hard plaster (o lder house). Yes, I got the ladder out and checked all the rooms.
So, climb into the attic, which was unfinished and had loose fill insulatio n, no noise. How can this be? Louder near the ceiling, but nothing in the attic?
Maybe it's coming in from outside. Nope. (quiet neighborhood, no traffic at all that time of night)
So I repeated the search several times, climbing ladders, going into the at tic, going outside and scratching my head, same results. Tired as I was, I couldn't let it go, couldn't think of anything else to try except keep doi ng the same thing that wasn't working.
Did I mention I'm right handed? I have a couple left handed kids but I am strongly right handed.
So when I'm standing outside thinking about it, I'm scratching my head with the right hand. And wondering why there is zero noise outside.
AND THEN: I scratch my head with my left hand. EUREKA! My digital watch had malfunctioned and was emitting the tiniest whine, impossible to locate but easy to hear in the dead quiet of night, especially when reflected by a hard plaster ceiling, and easily absorbed in the attic or outside.
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On 10/24/2014 8:55 AM, TimR wrote:

was emitting the tiniest whine, impossible to locate but easy to hear in the dead quiet of night, especially when reflected by a hard plaster ceiling, and easily absorbed in the attic or outside.

One morning, I put in my hearing aids, and there was a loud high pitch noise. Took me a while to find it. Teakettle blowing steam. I'd put it on for oatmeal.
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On 10/23/2014 12:01 PM, songbird wrote:

after I had a new branch duct run from the plenum to more effectively cool/heat a refinished downstairs space. I finally tracked it down to a damper in the 12" square duct which I had adjusted but hadn't tightened down afterward. Honestly, I never thought that the wing nut on the lever did anything.
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A switch on the thermostat to turn the fan on counts as a manual swiwth to most of us folks. I was trying to find out if the noise was there whenever the fan was running, or if the noise was there ony whenthe fan was running due to heated or cooled air. Heated or cooled air would change the dimens ions of the metal air plenums, that might start or stop the noise. You mu st investigate all possibilities.
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snipped-for-privacy@sbcglobal.net wrote:

of course, :)
next time i get moving again i'll see if the noise is there when the furnace switches on but _before_ the blower comes on. my guess is that it will be only when the blower is on because not much else is happening for a few moments while the heat exchanger is heating up.
songbird
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songbird wrote:

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EXT wrote:

it is outside the house and the pipe is insulated with spray foam and caulk. i will have a listen out there when get a chance in the next week or so. right now gardening is kicking my ass for the next few days.
songbird
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JAS wrote: ...

i did this before and the noise was there that time.
this morning the heat came on and i stood there and listened again to make sure that the noise is only there when the blower comes on. yes, that is the case. which is what i suspected before, but wanted to verify.
songbird
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