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Old 06-20-2013, 05:31 PM   #1
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Noisy blower only when AC is running


There is a loud noise coming from the blower area, but it only happens when the AC is turned on. Not hearing it with the blower or heater running. I can't pinpoint where it's coming from. Sounds like it's right inside the blower itself, but I'm not sure how that could be.

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Old 06-20-2013, 05:35 PM   #2
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Noisy blower only when AC is running


Metal duct on return flexing?

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Old 06-20-2013, 05:44 PM   #3
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Noisy blower only when AC is running


Not quite sure what you mean, but it happens as soon as it kicks on. At first I thought it was the motor going out since that's what it sounded like, but it sounds fine with the AC off.
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Old 06-20-2013, 05:55 PM   #4
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Noisy blower only when AC is running


a/c blower speed is fastest. does it sound like a grinding noise? it's very possible with the blower motor and attached blower wheel sitting inside of a blower CAGE that the wheel has slipped a bit. it'd be a metallic grinding sound if that were the case.
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Old 06-20-2013, 07:36 PM   #5
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Noisy blower only when AC is running


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Not quite sure what you mean, but it happens as soon as it kicks on. At first I thought it was the motor going out since that's what it sounded like, but it sounds fine with the AC off.
If you have a metal return it could be one of the sides flexing making a pop noise on startup. My sons does this. It's not correct but we know what it is.
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Old 06-20-2013, 07:42 PM   #6
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Noisy blower only when AC is running


I would almost agree but I won't ONLY if saying the return air metal can IS the problem. It is not. return air side of a system is low pressure. the lack of supply air causing the system to restrict air flow across the blower and evaporator and supply plenum and thus push out a return metal can would be the cause in this case, me thinks (me knows), high static pressure on the supply side. unless of course you don't have enough return air via duct and/or grill, then that could also be the cause of a return air can, like sucking through a straw and then sucking through the same straw with it half pinch closed.

I've also seen cabinets pop.
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Old 06-20-2013, 08:02 PM   #7
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Noisy blower only when AC is running


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Originally Posted by cddzero View Post
There is a loud noise coming from the blower area, but it only happens when the AC is turned on. Not hearing it with the blower or heater running. I can't pinpoint where it's coming from. Sounds like it's right inside the blower itself, but I'm not sure how that could be.
If it is coming from the blower compartment on the furnace, when was the last time the "Squirrel cage" had its fins cleaned off?
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Old 06-20-2013, 08:31 PM   #8
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Noisy blower only when AC is running


we need a better description of the noise. I'm almost wanting to think it's the noise of the return air grill, restricted as it's too small.
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Old 06-20-2013, 09:06 PM   #9
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Noisy blower only when AC is running


put the stat on FAN ONLY then sit down at the furnace and cycle the service switch o the side try to catch the noise....with it off grab the inside of the squirrel cage see if it goes back and forth...spin both direction is it free or brake hard to stop ...are the fins on the squirrel clean...
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Old 06-21-2013, 06:30 AM   #10
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Noisy blower only when AC is running


Look at the squirrel cage fins real close. If it is slipping from the hub there will be little scratch marks where they connect. That makes a noise on start up until it gets worse then it squeals all the time.
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Old 06-21-2013, 10:04 AM   #11
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Noisy blower only when AC is running


Thanks for all the speedy responses. I ran out of time and called a pro. Indeed it is the blower coming apart and was only heard on AC because of the higher speed as someone above had mentioned.

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