[700rpm]

I would suggest wiggling everything outside of the engine first, to see if something is loose. Sounds travel all through Model As and are really hard to pinpoint. Next do as Franklin suggests. Dropping the pan is an easy operation, even with engine pans. When you open it up you can quickly determine whether you have a problem in the low end, and what it is if it’s there.
I had an annoying rattle that sounded like it was in the engine, and was ready to open up it up, but before I did that, with some careful detective work I found it was actually a loose radiator support rod.