Whining Rattling Noise Underneath

Hi everyone. I have a 2005 access cab 4 cylinder 4WD with five speed manual transmission. There's a whining sound underneath under certain conditions that doesn't make sense to me. It's 100% repeatable. In 4th gear at ~38mph, a whining sound emitting from underneath near transmission area and comes and goes. It only happens under load or accelerating. It won't happen if your coasting from 40+ mph down to 38mph. It won't happen in 3rd or 5th gear at that exact speed either. Sound comes when truck is either cold or hot. Everything is stock including suspension, tires, motor, intake, etc. etc and the A/C is not on. I checked the obvious like heat shields but it sounds like a "stalled motor" like from a food blender for example. It doesn't make sense why noise is dependent on speed and gear selection combined. I may check level of the transmission fluid next but any ideas would be helpful. Thank you.