Guardian Furnace blower making noise after being on for a few hours

I have a Guardian GF8100D20MU11B furnace that has been making a noise that sounds like the squirrel cage fans are off-balance and grinding against the inside of the housing.

What's perplexing, though, is that it only occurs when the fan has been cycling for a few hours - it's initially is very quiet, though you can hear a faint knock. As time goes on, it gets worse and worse, until you hear a cycling scraping sound.

I have taken the assembly out, and it turns freely with no noise or contact on the fan housing by the fan blades. It is relatively clean inside, with no foreign objects. I re-adjusted the blades so that they were equidistant between both sides of the fan housing. There is a faint amount of wobble on both sides, but, again, there is no scraping sounds when I spin it by hand.

I placed the assembly back into the furnace and started it back up. You can hear a faint "tick tick" sound as the motor runs - but it doesn't get BAD until it's been on a few hours - in this case, I installed the assembly at about noon, and at about 10PM, I decided to turn the furnace off due to the noise getting bad.

My question is two part. One, at this point, does this sound like a motor problem? Because it sure sounds like it to me. (As the motor gets warmer, something gets out of whack, causes the assembly to go off balance.) I found the motor for about $150 and can replace it myself, because my local HVAC guys want about $700... ( Part # 024-34491-000 )

Two, I have a 2000sqft tri-level house with 14 vents, and three large returns....I've been told by two contractors that the furnace I have is way too big for the house. It's 100,000 BTU, and I live in Denver. This is the second fan issue I've had since moving in, and I'm told that due to the ducting I have, the motor is probably freewheeling. I replaced the original motor about 2 years back as it had overheated and seized. (Likely me using anti-allergen filters, though I have to wonder about this as well.)

Should I just pay someone to install a new furnace? Keep replacing motors as they burn out?

Thank you!

submitted by Attila-The-Pun
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