whining, whistling alternator since replacing it

I recently replaced the alternator with a second hand one. It looked quite new and the pulley spun easily.

From first start up it made a whining, whistling noise which gets quite loud at just under 3000rpm, then drops away at about 3200rpm. I've only driven 20 miles since replacing it, so wondering if it will settle down.

No battery light on, cars starts fine and the battery was showing 13.2 volts when I tested it with the engine going, which didn't rise as I increased rpm.

Should I keep driving it or does this sound like the alternator is faulty?