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Unless this is a really old unit the blower would be direct drive so no belt. But the blower could have a failed bearing. Sound could also be coming from the small draft inducer fan on the exhaust. If it's the inducer that's making the noise you need to replace it ASAP since an inducer failure would shut down the furnace. If the noise is a not very loud sort of whistle then this could be due to a gas pressure issue.
Inducer replacement is best left to a pro as is adjusting/dealing with a gas pressure problem. No test for this other than listening. A trained ear would be better able to pinpoint the source of the noise.
If it was a fan bearing in the inducer it might quiet down after the furnace get hot. Given the time of year I'd have a pro check this out while it's still running rather than wait until it shuts down and you have no heat.