Another, more informal equivalent of 'sup', is "τι λέει;" (meaning of course 'what's going on'). Now, about 'brr', in written language, we use the exact interjection "μπρρρ" (more 'r's' = more cold :P ), but while we talk, I don't think we use it. We say one-word phrases, like "κρύο" (cold)...