Might be a bad diode, or two. Alternators generate three phase AC, and use 6 diodes to rectify. Loss of a diode, reduces power output, and makes the output bumpy, hence noise and strobe.
Don't know why I did not mention that, agree completely. With the car running at fast idle, turn on the headlights, take your multimeter and switch to AC volts. That's RIGHT, "AC" and NOT "DC"
Read from alternator output stud to the engine block and post back here. It depends on "who you read" but more than 1/2 volt is darn sure high