Ok, Is the sound like heavy drumming/thumping from the hump area of the floor or a real metal to metal sound? It could be a bad transmission mount (easy fix) that could be letting the transmission shift around or up and hit the floor on heavy acceleration and also letting the u-joint hit the floor. If this is the case, I would look at your motor mounts too. Those bisquit type mounts can easily split and look fine until you put some load on them.
Other than that, I'm not to sure of the sound unless you describe it a little more.