[strictly]

I just had a failed alternator. It killed the battery overnight. I thought i had just left the lights on, and put another battery in.

Later that day while driving, i pulled the lights on (with the other battery) and a sound like pistons hitting valves/chains screamed out of the engine.

It scared the life out of me. Lucky for me it, turned out to be the alterntor diodes had shorted out and it was only noticeable when the lights were pulled on, and it had to really up its output.

Took the belt off, spun it over, engine sounded fine. Replaced alternator, no more scary noises! lol.