People track when talkers say 'uh' to predict what comes next. Speakers tend to say 'uh' before uncommon words ('uh… automobile') rather than common words ('car'). In a new eye-tracking study, researchers show that listeners use this information to predict an uncommon word upon hearing 'uh.' Moreover, when an 'atypical' speaker says 'uh' before common words ('uh… car'), listeners learn to predict common words after 'uh' -- but only with a native speaker. Source 5k.
I automatically say "Uhh.." When I'm not quite sure about something, especially when the memory is suddenly just wiped off my head as I'm talking about something. I never knew this has to do with being a native speaker or the languages you speak.