babble vs let on

babble is a noun but let on is not a noun.

Word	Noun	Adjective	Verb	Adverb
babble	Yes	No	Yes	No
let on	No	No	Yes	No

As verbs, let on is a hypernym of babble; that is, let on is a word with a broader meaning than babble:

babble: divulge confidential information or secrets
let on: make known to the public information that was previously known only to a few people or that was meant to be kept a secret

Other hypernyms of babble include break, bring out, disclose, discover, divulge, expose, give away, let out, reveal, uncover, unwrap.

babble (noun)	let on (noun)
gibberish resembling the sounds of a baby

babble (verb)	let on (verb)
to talk foolishly	make known to the public information that was previously known only to a few people or that was meant to be kept a secret
divulge confidential information or secrets
utter meaningless sounds, like a baby, or utter in an incoherent way
flow in an irregular current with a bubbling noise