A little belt slap when engaging the drive would be expected but not all the time. Is it Personal Pace or Variable Speed?

it is the personal pace. i am getting the belt slap the whole time during mulching. is there some sort of belt guide hook of some kind to keep it on track?

If it is really a rattling sound then I think it is something other than belt slap. I'm not familiar with your mower, but on other RWD Toro mowers there is a plastic cover over the belt drive. Remove that and see if you can see what is rattling.

There are some screws on the top of the deck that when removed allow the plastic belt cover to be removed from the bottom but I think all you'll see when you do that is a loose belt. When you're mowing, the personal pace lever pulls a cable that pulls a spring that tightens the belt. The faster you walk, the more tension on the cable, the spring and the belt. The rattling you hear is likely the belt or the gearbox. Mine does the same thing but it works like a charm. The attached link may be helpful. It's not quite the same model but should give you some insight.

Here is a link that might be useful: Toro Belt Replacement

That's a cool link - thanks from me too.