Fan Dose Not Stop Running When Closing The Door.

That's typically normal operation. The microwave has a thermostat which turns on the hood vent exhaust fan at a certain temperature to protect the controller.
Your owner's manual will cover vent fan operation.
If for some reason you don't have yours, you can download Whirlpool owner's manuals and installation instructions here.

I always recommend that you use the vent fan whenever you're cooking on the stove top to keep heat, steam, and grease out of the controls and door panels.
If - above and beyond this - the fan has a problem, that could make it excessively loud. Some are just loud though, it's hard to judge without actually hearing it.
You can contact Hotpoint customer service with your model number to the CS # in the back of the manual and ask them about the loudness. They may be able to tell by listening over the phone if it's abnormally loud.

I hope this is helpful to you!

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