Transmission Issue

So I was backing my truck up to my camper, I put the trans in reverse and pressed the accelerator and the truck did not move nor did the truck rev. I thought that was odd, I put the truck in Drive and the same think happened, it didn't move nor rev. I did this a few times to see if I could get the truck to move and twice I heard the trans gears grind. So, I put the truck in 4WD to move the truck, put the trans in L and the truck moved forward with ease. But then I heard a whistling noise. Put the truck in park, disengaged 4wd and the whistle is still there. The whistle is only there when the truck is idling in drive and in park. My truck is an auto. Anyone have any ideas. My truck is a 13 with 54000 miles. I did turn the truck off foe about an hour, then hopped back in and took it for a ride. The trans shifted fine and the truck ran fine, however the whistle is still there. Sounds like a very loud vacuum leak, but I can't find anything.

 

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