I'm familiar with that sound. What you're hearing is the torque converter bolts hitting the webbing on the engine block. If you check your torque converter bolts, you'll probably find out that they are loose. I've done a lot of research online over the years trying to figure out why some people have this problem and others don't. You should try using Loctite and re-installing the bolts. If that doesn't work, you might have to have the bolts drilled and install lock wires to keep the bolts from backing out. That's what I did and it was the best thing I could of done to fix the problem.