death knell vs death bell

death knell and death bell both are nouns.

Word	Noun	Adjective	Verb	Adverb
death knell	Yes	No	No	No
death bell	Yes	No	No	No

As nouns, death knell and death bell are synonyms defined as:

death knell (noun)	death bell (noun)
a bell rung to announce a death	a bell rung to announce a death
an omen of death or destruction