Alex is right...driveshaft isn't moving in park.
...and not to argue with you, but it's very hard to figure out where a sound is coming from. If it's definitely not under the hood (you should listen with a stethoscope or hose), then the only other things I can think of is fuel pump. There's nothing else moving when you're in park.
Try filling up the car above 1/2 full and see if that quiets the noise down (if you happen to be low on fuel)...
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