Hey Yawk, Welcome...
As for your question, the difficulty of what you want to do is greater than you may think, at least for most baits. You will be far better off putting rattles in after injecting (especially for consistency), whether piercing a hole with a nail as ctom suggests (for smaller rattles) or coring with a small tube/needle (for bigger rattles).
If you are bent on trying insert rattles while injecting, the craw mold might be one of the best molds because of the matching appendages, BUT your rattles will have to span the body cross-wise to be held in the middle of the cavity... and of course the legs or claws may not fill in correctly because of the rattles could block the flow plastisol during injection.
The one way around this would be to buy matching craw and claw molds from Do-it... first inject your rattles in the claws alone, then put these rattle claws in the mating body mold and inject that... this will give a nice product but it will be a labor/time intensive way to accomplish what you are looking for.
My recommendation would be to put them in after injection.