I fear I am at once Geta and his grammarian. For installment number three of ancient Greek animal sounds, it is time for some swine.
Historia Augusta: Geta 5.4-5 (thanks again to @Stevendsmith74)
“It was [Geta’s] habit to pose questions to grammarians, for instance, how they might name the way various animals make sounds: sheep bleat, pigs grunt, doves coo, boars grunt, boars growl, lions roar, leopards sneer, elephants sound horns, frogs croak, horses whinny, donkeys bray, bulls low. He would prove each of these with ancient writers.”
Familiare illi fuit has quaestiones grammaticis proponere, ut dicerent singula animalia quomodo vocem emitterent, velut: agni balant, porcelli grunniunt, palumbes minurriunt, porci grunniunt, ursi saeviunt, leones rugiunt, leopardi rictant, elephanti barriunt, ranae coaxant, equi hinniunt, asini rudunt,1 tauri mugiunt, easque de veteribus adprobare.
It seems likely to me that Ancient Greek pigs said γρῦ γρῦ
“goggrusai: to make noise like a pig”
γογγρύσαι· ὡς χοῖρος φωνῆσαι
“The noise of a pig
γρύλλη· ὑῶν φωνή
Cf. Photius gogggruzein and grulizein: “swine sounds”
Γογγρύζειν καὶ γρυλίζειν· ἡ τῶν ὑῶν φωνή. [cf. Zonaras]
Schol ad Ar. Pl. 22
“Who says “oink”—this is either from the sound of pigs or from trash [grutê, small bits, inconsequential things].
… ὃς γρῦ λέγεται· ἢ ἀπὸ τῆς τῶν χοίρων φωνῆς ἢ ἀπὸ τῶν γρυτῶν·
Zonaras does not seem to agree completely, but he does have: “grullos, a pig”. Γρύλλος. ὁ χοῖρος
Later, he says, “gru: something brief. a chance. Some say it is the filth beneath a nail or a type of small measure.”
Γρῦ. τὸ βραχὺ, τὸ τυχόν. ἔνιοι δὲ τὸν ἀπὸ τῶν ὀνύχων ῥύπον ἢ εἶδος μικροῦ νομίσματος
Zenobius Sophista, 5.54
“Among Attic speakers gru is used to describe something small and accidental. For they call both the dirt under a fingernail gru and the bric-a-brac of a home grutaria. The man who sells the bric-a-brac is a grutopolos.”
Εἴρηται δὲ καὶ παρὰ τοῖς ᾿Αττικοῖς τὸ γρῦ ἐπὶ τοῦ μικροῦ καὶ τοῦ τυχόντος. Καὶ γὰρ τὸν ἐν τοῖς ὄνυξι ῥύπον λέγουσι γρῦ, καὶ γρυτάρια τὰ κατὰ τὴν οἰκίαν λεπτὰ σκευάρια, καὶ γρυτοπώλην τὸν τὰ σκευάρια πωλοῦντα.
This and the Suda preserve the proverb “Dion’s Grunt”, for something small and incidental” Τὸ Δίωνος γρῦ, ἐπὶ τοῦ μικροῦ καὶ τυχόντος
Here’s Beekes on this