death bell vs death knell

death bell and death knell both are nouns.

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

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

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