is it normal for my rat to chew randomly when there is nothing in her mouth? cookie my youngest rat does that but my other rat daisy, does not.
is this normal?

Yes! It's called bruxing, means she's happy! Or stressed... but usually, unless she's trapped in a cone or being force fed meds, it's happy.

I think of bruxing like a cat purring. It's a rats way of purring. :)

Sometimes, is it proper mouth chewing movements or bruxing (grinding teeth)?

ok thanks! :)