If Torah is the source of morality, defining what is right and wrong, then Pre-Matan Torah, pre-Sinai, what form of moral objectivity existed?
Apart from the Noachide laws mentioned already, there are a few references in the Torah itself to a shared moral code.
וַיִּקְרָ֨א אֲבִימֶ֜לֶךְ לְאַבְרָהָ֗ם וַיֹּ֨אמֶר ל֜וֹ מֶֽה־עָשִׂ֤יתָ לָּ֙נוּ֙ וּמֶֽה־חָטָ֣אתִי לָ֔ךְ כִּֽי־הֵבֵ֧אתָ עָלַ֛י וְעַל־מַמְלַכְתִּ֖י חֲטָאָ֣ה גְדֹלָ֑ה מַעֲשִׂים֙ אֲשֶׁ֣ר לֹא־יֵֽעָשׂ֔וּ עָשִׂ֖יתָ עִמָּדִֽי׃
Then Abimelech summoned Abraham and said to him, “What have you done to us? What wrong have I done that you should bring so great a guilt upon me and my kingdom? You have done to me things that ought not to be done. (Bereshit 20.9)
This morality is often linked to fear of God. Hence Avraham's answer:
וַיֹּ֙אמֶר֙ אַבְרָהָ֔ם כִּ֣י אָמַ֗רְתִּי רַ֚ק אֵין־יִרְאַ֣ת אֱלֹהִ֔ים בַּמָּק֖וֹם הַזֶּ֑ה וַהֲרָג֖וּנִי עַל־דְּבַ֥ר אִשְׁתִּֽי׃
“I thought,” said Abraham, “surely there is no fear of God in this place, and they will kill me because of my wife. (v.11)
This is also the reason given for the midwives who refused Pharaoh's execution orders, and also Joseph's refusal of Potiphar's wife's advances:
וַתִּירֶ֤אןָ הַֽמְיַלְּדֹת֙ אֶת־הָ֣אֱלֹהִ֔ים וְלֹ֣א עָשׂ֔וּ כַּאֲשֶׁ֛ר דִּבֶּ֥ר אֲלֵיהֶ֖ן מֶ֣לֶךְ מִצְרָ֑יִם וַתְּחַיֶּ֖יןָ אֶת־הַיְלָדִֽים׃
The midwives, fearing God, did not do as the king of Egypt had told them; they let the boys live. (Shemot 1.17)
אֵינֶ֨נּוּ גָד֜וֹל בַּבַּ֣יִת הַזֶּה֮ מִמֶּנִּי֒ וְלֹֽא־חָשַׂ֤ךְ מִמֶּ֙נִּי֙ מְא֔וּמָה כִּ֥י אִם־אוֹתָ֖ךְ בַּאֲשֶׁ֣ר אַתְּ־אִשְׁתּ֑וֹ וְאֵ֨יךְ אֶֽעֱשֶׂ֜ה הָרָעָ֤ה הַגְּדֹלָה֙ הַזֹּ֔את וְחָטָ֖אתִי לֵֽאלֹהִֽים׃
He wields no more authority in this house than I, and he has withheld nothing from me except yourself, since you are his wife. How then could I do this most wicked thing, and sin before God?” (Genesis 39.9)
There are more examples of appeals to a shared common moral code, but to return to your question, the source of morality seems to be a relationship to God, rather than the rules of the Torah.
The simple answer is that morality can exists and can be derived without the Torah. See for example Eruvin 100b:
אמר רבי יוחנן: אילמלא לא ניתנה תורה היינו למידין צניעות מחתול, וגזל מנמלה, ועריות מיונה. דרך ארץ מתרנגול
Rabbi Yohanan said: Even if the Torah had not been given, we would nonetheless have learned modesty from the cat, which covers its excrement, and that stealing is objectionable from the ant, which does not take grain from another ant, and forbidden relations from the dove, which is faithful to its partner, and proper relations from the rooster (Sefaria).
It seems that human intuition guided by observation of the world would be sufficient.