Lights flicker and gauge jumps around???

If no whirring, there's always modern cost cutting stator winding methods, which are called high current, because no one would buy alternators honestly marketed as
"Low Voltage at Idle Speed".
Proper Stator windings have 12-13 winding loops in each pole, and can be estimated quickly looking at the windings through the slots in the case. If yours has only 6-7 loops per pole, the voltage regulator can send the full voltage to the rotor trying to maximize current flow in the rotor to increase output, and still not create enough to charge at idle rpms.