A heavily slowed down **dog bark** could be a start, but you need some power with something like a **cow/bull**. Some sort of **hiss/wind** as well, because this thing must project quite a lot of air! An **elephant** could help and why not a **siren** (WW2 kind of bombing alert).

This is just brain storming, I might come back with more! :)

EDIT: revving up an **engine** could add character to the roar?