One of the pulleys that the serpentene belt rolls on is VERY loose and makes a lot of rattling. How do I tighten or fix that?

Well, replacement is the "cliff'* notes" version. A pulley, to my knowledge, isn't adjustable. The bearing is worn and needs replacing. It is possible that it isn't an idler pulley, but the belt tensioner. Either way, they should be replaced. Not too big of a job if you aren't afraid to pull out the wrenches.

Pop off the belt and check the belt tensioner, easy enough job. Keep us up to date

ken