really vs rattling

really is not a noun while rattling is a noun.

Word	Noun	Adjective	Verb	Adverb
really	No	No	No	Yes
rattling	Yes	Yes	No	Yes

As adverbs, really and rattling are synonyms defined as:

Other synonyms of really include real, very.

really (noun)	rattling (noun)
	a rapid series of short loud sounds (as might be heard with a stethoscope in some types of respiratory disorders)

really (adjective)	rattling (adjective)
	quick and energetic
	extraordinarily good or great; used especially as intensifiers

really (adverb)	rattling (adverb)
used to give emphasis	used to give emphasis
in fact (used as intensifiers or sentence modifiers)
in accordance with truth or fact or reality
in actual fact