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Hello, I'd start with pulling the blower motor out and inspect it. Could be something in there as well. You'll find the blower under the dash on the passenger side. It should be just behind the cover on the bottom.
Pull it out, check for debris in the heater box, and it can't hurt to turn it on while holding it to see how the motor sounds on it's own. While WD-40 my quiet it temporarily, I would recommend replacing the motor to ensure the repair. I also would worry about wd-40 causing an odor.
If you need any further assistance, please let me know!