The oil was changed with synthetics about 2500 miles ago.Today,I jacked it up and got under and it seems to be the catalytic converters which you can see a picture of at
http://www.********************************************************************************************/images/catalytic/MB2256.jpg . It is the thing that has 2 ovals side by side.It stops rattling for a little when you tap it and then it starts again.
So,hopefully that's all it is.