loud vs loudly

loud is an adjective but loudly is not an adjective.

Word	Noun	Adjective	Verb	Adverb
loud	No	Yes	No	Yes
loudly	No	No	No	Yes

As adverbs, loud and loudly are synonyms defined as:

Other synonyms of loud include aloud.

loud (adjective)	loudly (adjective)
characterized by or producing sound of great volume or intensity
(used chiefly as a direction or description in music) indicating that said piece or segment should be played loudly
tastelessly showy

loud (adverb)	loudly (adverb)
with relatively high volume	with relatively high volume
	used as a direction in music; to be played relatively loudly
	in manner that attracts attention