There is a pronounced growl when I accelerate?

2005 Subaru 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 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 everything seems okay. The 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