We moved into a house about a year ago. It has a medium-efficiency gas-fired hot air furnace that's probably about 17 years old. Our house inspector pointed out that the inducer fan was very noisy when running. He predicted that the motor bearings were probably worn and the fan needed replacing.
But other than the noise, it seemed to work OK, so I didn't get around to looking at it until recently. Although the inducer fan motor is a simple shaded-pole design, it seems to be equipped with ball bearings instead of bronze sleeve bearings, and they still seem to be in good shape. So I removed the fan assembly from the furnace to take a look.
The actual fan is a squirrel-cage type about 4 inches in diameter and
1.5 inches thick. Every blade on that fan had greyish deposits caked on it. Because the deposits were uneven, the squirrel cage wheel was out of balance. There were enough deposits that it probably reduced airflow by a few percent as well.I picked an object to use as a scraper, and went all the way around the fan wheel knocking off most of the deposits. Then I put the fan assembly back on the furnace. It is now *much* quieter. There's still some vibration, so the wheel still isn't perfectly balanced, but you don't hear it unless you've close to the furnace now.
An even better repair would be to buy a new fan wheel of the appropriate size, and just replace the old one. I've seen these for about $25 on ebay, so that's a lot cheaper than replacing the whole assembly including motor.
Dave