He doesn't open his mouth and the "rrrrrr" sound would be deep in his throat so not really rolling or vibrating. It's rather mmmmrrrrrr
deep in this throat and not reaching his mouth to come out. I don't know how to describe it better.
In a Cardassian library
or in a Cardassian gallery
"Reagan, it appears, is really only an ardent unionist if the unions in question are in Poland" - Stephen King, Skeleton Crew