When we flush the second floor toilet the pipes rattle. This just started. It is not the fill pipes because we tested by flushing normally & then dumping a pail of water into the toilet so no tank refill occurred. In both cases the rattle occurred during the time the water drained down the soil pipe, not the fill lines.
Any ideas? Is there some kind of attachment bracingof the soil pipe within the walls that may have broken?

Does it sound like the pipe itself rattling or something caught in the pipe banging around? This really bears further inspection. I suggest you pull your toilet to investigate further.

Chayn took the question right out of my mouth. Rather then the soil pipes rattling it could be something inside that you hear.
Did this just start or has it always rattled? Where is the noise the loudest? In the wall or nearer the toilet? Describe the noise. A deeper thumping? A little rattle? Let's have more details please. Tom