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If the unit is over 7 years old the bearings are probably going bad in the fan motor and allowing the blower wheel to touch the housing slightly... you can turn the power off to the furnace, access the blower wheel and see if you can detect such a clearance related problem or any up and down play at all in the fan shaft bearings.
If I miss you tonight, I will pick your response up in the morning.
Hello again, these days motor bearings are not easily available nor is it cost effective to replace them at todays labor rates... the entire motor is replaced in that case.
If you can tell me which direction you are from the center of the nearest city I can most likely find a few well rated shops nearby that can do that work for you.