Both are correct. "Loud" is what is known as a flat adverb, meaning that the adjective form ( without ''ly" )can function as an adverb. You can say either "loud" or "loudly" in this sentence.

Flat adverbs were commonly used several centuries ago, therefore, they are not considered to be incorrect. In modern times, flat adverbs are used fairly often, however, speakers who leave the -ly off the adverb may be considered less educated by some people. Students are usually taught in school that leaving the -ly off an adverb is incorrect. On a standardized English test, "loud" would be considered an adjective while "loudly" would be the correct adverb.

Lady Gaga belted out the song so loudly that we could hardly hear ourselves talk.