MozzEc - There is a distinct metal-to-metal clanking sound in the video. I highly doubt that is being caused by a rubber belt.
I concur with Donabed that if are having mechanics taking guesses, they aren't mechanics. Avoid.
Diagnose.
An inexpensive but effective stethoscope is to use a long metal screwdriver, extension bar or two put together, or what have you and put one end to your ear with the other end up against whatever you want to listen to. The sound will travel to your ear and you will be able to hear noises inside the engine, alternator, water pump, cylinder head, engine block etc. This can help you pinpoint the noise, instead of taking wild guesses and the very expensive, time consuming and wasteful parts replacement routine.
With a cold engine, removing the serpentine belt and running the engine for 30 seconds will tell you if the noise is coming from any of the accessories or not. After removing the belt and running the engine, if you don't hear the offending noise anymore, then it is one of the accessories that the belt was running. If the noise persists, then you know it is an internal engine problem. Don't run the engine for more than 30 seconds, as with the serpentine belt removed, the water pump will not be turning and the engine will overheat if you let it run for too long.
Again, is the oil being changed only once per year with this engine? Petroleum oils are not safely capable of that.
Get back with us and let us know what you find. Avoid discount auto parts stores at all costs as they will just encourage you to, "Here, but X and replace it and if that doesn't work then buy X and replace that...", etc., etc., etc. Lots of your money later vacumed out of your wallet later the problem still exists. Never do the parts replacement routine.