I remember my dad telling me one time that thunder was caused by two clouds colliding. I believed that for the longest time.
Never was sure if my dad actually believed that or was pulling my leg, and now I no longer can ask him and never thought to ask him when I could.
I just assume he was pulling my leg. Because as I got older the smarter my dad became.
Thus I conclude, rightly or wrongly, he must have know it was lightning that creates thunder from the sudden increase in pressure and temperature from lightning which produces rapid expansion of the air surrounding and within a bolt of lightning