Blower Motor Single "Stutter" when Condenser comes online
That's the sound I would associate with the Starting capacitor of the blower motor having trouble. It could be a number of things; the capacitor is failing, the switch that CONTROLS the capacitor is failing, the blower electrical circuit has a loose connection somewhere, or, and what I suspect is likely, by the time this blower turns on, everything else is ALREADY turned on and is causing a voltage drop, which is making the motor on this blower lose torque right as the centrifugal switch begins to function, then it slows down and causes that "stutter" effect until the blower gets to full speed. because heat rises, your upstairs unit will always come on first, so the total load is at its highest when the downstairs blower is called for, which would be AFTER the compressor is already running as well, meaning the blower is the LAST big load in line and has to contend with a voltage drop issue.