Transmission noise

I have a 2008 Mercury Mariner Premier with a 3.0 engine and all wheel drive. Today I had a noise suddenly start like something that was hitting as I was moving. I removed the rear driveshaft to see if the noise was still there in which is was. It sounds to be coming from the back of the transmission. You can only hear it when it is in gear and moving. I was wondering if anyone has had this issue or if anyone had an idea what it may be.