Is roar a noun?
Yes, the word 'roar' is both a noun (roar, roars) and a verb (roar, roars, roaring, roared).
We heard the roar of a lion in the distance. (noun)
The boss will roar when sees this mess. (verb)
The roar that you ask about is actually not a roar but a slightly less offensive sound called the loud yawn. In some cultures it is common to mistake the loud yawn for a roar in the book the roar, but most people realize that mistake too late and post that question on the internet before learning that they should have referred to the roar as a "loud yawn."