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When it comes to air conditioning systems,...
You may have a metal venting wheel out of balance on the motor. The 90% effieient furnaces have very corrosive exhaust fumes and after time slowly eat away these venter motor wheels. If you can remove the assembly, you will probably find this to be the culprit. usually you have to cut the shaft of the motor off to get the wheel off. Let me know if you need more info.
This is vented in plastic correct? when the motor is running, gently rest your hand on it and see it the vibration stops.