I had a noise similar to that. I used to do it all the time. I found all the bolts that hold the flywheel to torque convertor bolts loose, and one of them wasn't even seated all the way.

I have a hunch that the flexplate is cracked though because when I put the car into reverse I get the noise still.

Funny thing is, I had the tranny replaced last year, and the noise started soon after.

Good Luck.