Loud can be both an adjective and an adverb, but loudly may be used only as the latter. It is permissible to say "Shout out loud" or "Shout out loudly." Such optional usage is common after such verbs as sing, talk, say, scream, and laugh, but among careful speakers and writers, loudly is preferred. Also, one "boasts loudly (not loud)," and loudly is accepted idiomatic usage with such other verbs as insist, brag, state, and exclaim. It is safer to use loud only as an adjective (a loud noise, the noise was loud).
Dictionary of problem words and expressions. Harry Shaw. 1975.