Ringing from transmission?

2005 2500hd Silverado LLY Duramax
When ever i shift from park to either drive or reverse there is a pretty significant ringing noise that lasts about 1 second on the dot everytime, due to the fact that i have to be in the truck to shift it, i cant be under the truck to listen for where its coming from, it sounds like its coming from the center of the truck, any ideas on where to start looking? I dont want to just start replacing parts that arent bad.

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