If it's a newish belt it might not have been fitted properly. Release and lock off the auto tensioner and check the manual tensioner still takes up the belt slack. If the belt is loose at this point take up any slack by adjusting the manual tensioner, then retighten the manual tensioner and release the auto tensioner. I'd suggest checking for any play or roughness in bearings while you're there.
If that doesn't solve your problem it's probably your crank pulley on its way out, they're usually pretty damn noisy - lots of videos on youtube to give you an idea.
If that doesn't solve your problem it's probably your crank pulley on its way out, they're usually pretty damn noisy - lots of videos on youtube to give you an idea.