I found the culprit. Stretch stitches go back and forth. So the feed dog reverse mechanism kicks in at appropriate intervals to accomplish the back motion in the stretch stitch. There is a spring that keeps the feed dogs from staying in reverse. When the spring releases and allows the mechanism to come back forward, it makes a clattery noise.
Here is a photo of the noise maker looking down into the machine
And here is one looking at it from the end and a little below.
I oiled this mechanism and ran the machine and eventually it quieted down. It sill clacks some but now it does not sound as if something is going to shatter. The magic of sewing machine oil.
This machine is belt driven. Double belts. This is a photo of the 1803. The machines are very similar and the only difference between this and the 1813 is the handwheel mechanism.
I needed to loosen the screw holding the motor in place. It is actually a slotted nut. Lucky for me I had a new socket set.