sneak in vs insert

sneak in is not a noun while insert is a noun.

Word	Noun	Adjective	Verb	Adverb
sneak in	No	No	Yes	No
insert	Yes	No	Yes	No

As verbs, sneak in and insert are synonyms defined as:

Other synonyms of sneak in include slip in, stick in.

sneak in (noun)	insert (noun)
	(film) a still picture that is introduced and that interrupts the action of a film
	(broadcasting) a local announcement inserted into a network program
	an artifact that is inserted or is to be inserted
	a folded section placed between the leaves of another publication

sneak in (verb)	insert (verb)
insert casually	place, fit, or thrust (something) into another thing
enter surreptitiously	insert casually
	put or introduce into something
	fit snugly into