Odd "whirring" noise

Hey there everybody,
I have owned an 07 since 2015. In April I bought a 24 Taco and while cleaning the 07 up inside and out, I went to move the 07 back into the driveway, and when I started it, I noticed a "whirring" noise coming from under the hood. It seemed to stop for a few minutes but came back. The noise matches the RPM of the truck - so it changes when selecting a gear and when it shifts, and when in park and neutral. It does seem to settle after the initial RPM surge on start up. I took it to a mechanic that I have been using about 15 years and he seems to think it is in the transmission. I dont think so, but I am not a mechanic. The truck has 240k on it and still runs great, but I am trying to sell it since I didnt trade it in and dont want that noise to scare off a buyer. Any thoughts?
EDIT: Although I didnt not wash the engine compartment, it did happen after water wash splashing around - I feel like if it were water somewhere it would have dried up by now.