Yup, sounds like a missing gear tooth.
Does it only jump in one gear?
Not that it's a solid indication, but it's a good way to troubleshoot.
A broken gear tooth can easily float around and break other gear teeth too.

Go to the store and buy a funnel with a screen at the bottom (or make something that can filter metal particles out of the oil) and drain it.
If you have a magnetic drain plug you'll probably find parts of it right away.
Run a magnet all over the bottom of the case to pull the broken bits toward the drain.
Go from the outside and slide a STRONG magnet straight toward the drain.
Alternatively, if you can fit a tiny magnet into the drain with a flexible shaft, you might be able to pick it up.
The point is to determine for sure it's a gear tooth, and a particles or teeth are a smoking gun.
Either way, it sounds like you're going to be splitting cases or sending it off for a rebuild.