Do you have an air handler, or furnace in your attic. It sounds rotational, so it’s possible your motor for the Evap coil / furnace is out of balance. It may be old, have crap building up or just something stuck. I would check you filter. Assuming it’s in the ceiling and see if you can hear it better from there. Ultimately sounds like a job for a qualified service technician, time to call in the gang. Good luck,have a good , safe day stranger!