2002 Engine Rumble 40-45 mph

I've got a 2002 Chevy Tahoe LT that makes a low grumbling noise when accelerating gently during 40-45 mph, just after shifting (automatic) into 4th gear. Sounds like a diesel vehicle sound. Dealer hasn't been able to determine what the noise is, but the technicians I drive with can sure hear it. Any advice or other sites to post this on would be greatly appreciated! Thanks for your time.