Milkweed Beetles also eat Milkweed. Their damage pattern is a little different than that of Monarch cats. Milkweed Beetles will eat large, jagged damage patterns towards the end or on the side of the leaf, whereas young cat instars will eat crescent shaped to round holes in the middle of a leaf. Monarch cats are good hiders as well, preferring to nestle down in the crown or hang out on surrounding vegetation when not eating. Larger instars will cut the midrib of the leaf to cut the flow of sap and then eat from the tip up.
Milkweed beetles will also eat the entire midrib out, leaving the entire leaf to curl and die.

There are also those cute little fuzzy caterpillars Milkweed Tussock moth I think? But they can be quite voracious and defoliate a plant if there are a lot of them.

Another possibility is Milkweed Bugs that look like Boxelder Bugs. I have both of those critters now.

These insects are usually visible, but may be hiding under leaves or drop off the plant if disturbed.

Here is a link that might be useful: Milkweed bug

Thanks Leafhead and bernergrrl!

So, I went out and found a milkweed beetle, and they do make a noise if you hold them--learned that from the link I posted. It is incredible and very cool.

I will show some tolerance for them. .. :)

I hear a clicking noise when I pick up a tomato hornworm. I'll have the try picking up a milkweed beetle.