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Old 11-25-2021, 08:58 PM   #1
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Anyone got an idea on this odd furnace fan noise?

It sounds like something rubbing the squirrel cage fan, but only when it's cold. When I fired it up for the first time this year it just howled and howled, a steady noise. When I realized it wasn't lighting and turned the propane on, ahem, it fired as normal once the propane started feeding. As it warmed up the noise got lower and lower in volume until it cycled off. Then when it cycled back on the noise was still there, just very muted and it finally faded completely. Thereafter that evening it sounded like normal. I remember it did this some last year but never realized it was directly temperature related.

When I first bought the coach the fan would run but it wouldn't light. After some troubleshooting and cleaning of various insect evidence I found the sail switch to be stuck off. After freeing it up I figured I'd try it and see if I needed to replace the switch or not. It fired but made the noise. My first thought was either my insect nest cleaning was inadequate or I had mucked up the sail switch so I opened it back up. Neither was an issue that I could tell. I could see no way the sail switch would actually contact the fan and it worked fine. Finding that the noise went away I skipped it.

Not a huge deal since it goes away but it still bugs me. What is it? It's been a ways back so maybe it was too warm for the issue when I took it apart? I have some other issues that should be looking at but this bugs me and I'm wondering if I need to crack it open again when it's cold for another look.
It would be nice if someone had seen this before and could save me doing more exploratory surgery. I can hope, right?
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Old 11-25-2021, 09:58 PM   #2
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Motor bushing
Room Air Wheel rubbing fan case until warmed up
Combustion Air Wheel rubbing fan case until warmed up

Those are the only 'moving parts' can can rub and make noise
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Old 11-26-2021, 05:53 AM   #3
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Sounds like a bad motor. I have had to replace one of mine.
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Old 11-26-2021, 09:27 AM   #4
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Sounds like bushings/bearings going bad.
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Old 11-26-2021, 10:39 PM   #5
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I didn't get the noise spinning it by hand last time I was in it. Maybe I could force it into running with the case open next time and get a shot at the bearings with a stethoscope? I've had bad fan bearings before and it was more of a squealing type noise than this. This is more of a loud whirr like a piece of paper might make against the vanes. I think. Wouldn't be the first time I've been fooled. Seems like if it was rubbing I would get some evidence of that when I spin it by hand. I'll get back at it soon I hope.
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Old 11-26-2021, 10:56 PM   #6
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What Brand/Model is furnace?

Older model numbers/low btu rated.....motor RPM 1100
Older model numbers/high btu rated....motor RPM 2200-2400

Can you spin it that fast by hand.....

DC to furnace RED wire (ON/OFF switch) is Continuous from fused source
RED wire from Time Delay Relay to Motor.....connect those two and Fan will Run full speed
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