Comment by Izkata

> Are you claiming that a rhotic "er" exists in American English, or that the British particle "er" is distinct from the British particle "erm"?

Neither. In Amrican English we're fully pronouncing the "r" when we say "er" instead of "um".

> Obviously people can hear the difference between vocalizing with your mouth open and vocalizing with your mouth closed.

That would be "uh" vs "um", not "er".