3isms

I have a clunk or flap coming from the underbody(or sounds like it) of my '00 loutback egacy wagon.....I thought it was a broken exhaust hanger and had it welded and that wasn't it.....I just bought it and the lady said it was the transmission but the thing runs fine in all the gears...its a AWD automatic......I have looked for anything loose and looked at the CV boots and the engine is purring like a kitty...HELP!