I think my transmission is making a winding noise

[The Beast 6.0 liter]

Well I took my truck on a 3400 mile trip. Well on the way back I had about 1800 pounds of weight in the back. I had absolutely no issues on the hole trip. Well after about a month of being back I noticed my truck started making a whining noise at an rpm range from 1200-1400. Now the Wierd thing is it will do it driving down the road or sitting in the drive way in park or neutral. I got under the truck and I can't tell if it's coming from the transmission or the exhaust. It is coming from either the transmission or the engine I'm guessing. If anyone could help me out that would be great. Also besides the noise the transmission works great. Also the noise goes away when im going up hill. Pretty much putting a load on the engine. My truck is a 04 f250 crew can long bed. Thanks for your help.