2005 Subaru Outback growling noise

My 2005 Subaru Legacy Outback, 2.5 with 4 speed automatic has a perplexing problem. As soon as I start to accelerate, there is a pronounced growl. At road speeds the growl gets really, really loud when accelerating, but quiets down at road speeds or when you release the gas pedal. I've checked the wheel bearings and axles, and the exhaust, but everythings seems okay. Fluid level is okay, with no leaks. I can hear a chatter coming from somewhere inside the transmission near the pan when listening through a stethoscope.