sinking vs sinking feeling

sinking and sinking feeling both are nouns.

Word	Noun	Adjective	Verb	Adverb
sinking	Yes	No	No	No
sinking feeling	Yes	No	No	No

As nouns, sinking and sinking feeling are synonyms defined as:

sinking and sinking feeling: a feeling caused by uneasiness or apprehension

sinking (noun)	sinking feeling (noun)
a descent as through liquid (especially through water)	a feeling caused by uneasiness or apprehension
a slow fall or decline (as for lack of strength)
a feeling caused by uneasiness or apprehension