Transmission noise

2002 Chevy Astro AWD 78000 miles

At 40+ mph when cruising, I get a rumble noise when accelerating or decelerating. It corresponds to when the transmission is either loading or unloading the engine. If I keep accelerating slightly, keeping the slight load on the engine, there is no noise. But when I let off the gas and decelerate back I get a second or so of rumble. I'm picturing some transmission or differential gears chewing themselves up. Been doing this the last few months that I recall. We bought it at 30000 miles and had 5 years with none of these symptoms or other trans problems.