回答“Why is the fan so loud?”In the iMacs I have worked on there is a temp sensor cable that runs from the hard drive to the logicboard. You are right that the hard drive itself does not have a fan but if the iMacs use a specific kind of hard drive and there is a thin cable that runs from the hard drive to the logicboard that is called a hard drive temp sensor cable. If that cable is missing it will cause the fan to run at high speeds. I learned about this while working at an apple store. Look it up.
回答“Why is the fan so loud?”I've seen this problem before and it was because the hard drive was replaced with a different brand but the hard drive sensor cable was not changed. Different brands use different temp sensor cables. That might be the problem.
回答“My Powermac g4 bombs but nothing comes up on my monitor”I would recommend checking to see if the cables are connected correctly. From what you've said it sounds like you need a new graphics card.