On page 14 the small text is just nuu with dakuten (the double dots) added. Of course neither nu or u have any readings with dakuten (ha goes to ba, ko goes to go, nu goes to ??? nothing). In that case you're supposed to take it as going beyond nu to something even more gutteral, so he's basically going 'nggh' in an extra feral way. You could translate it as GRRR or GROWL or NGHH or similar.