1999 Ford F-150 5.4 - FYI... I had issues with my truck pulling my camp trailer up into the mountains. The local Ford dealership said it was the alternator missing at the top-end. I replaced the alternator with one from Autozone, a few months later an annoying whining sound came from the engine compartment. The whining got louder as I accelerate. I slipped it into neutral, accelerated and still heard whining. The forums said it was the pulleys, so I replaced them. Whining was still there. Having so much fun under he hood I took the alternator back off, I couldn't even spin the pulley, bearings had gone bad. Took the alternator back to Autozone for a free replacement. New alternator and pulleys later, I have a strong quiet truck again.

 

Just a FWIW but so far I've replaced two 5.4 alternators, used the NAPA new part each time. Sure they cost well over $150 each but they've been flawless without a hiccup so far.

Glad your issue was solved so easily---those annoying noises can drive a person to absolute distraction!