Now if you want realy really loud, try unplugging a GPU cable from the engine nacel, which is only a metre or so behind the prop after its started.
One day I was training a newbie, his first day on the job. Never been around aeroplanes before, let alone walking to within 1m of a spinning prop. This time, the crew gives us the GPU disconect signal. Just as the newbie unplugs the GPU cable the crew lifts the props off the start locks. Poor newbie needed a change of underwear.
On their return, I firmly said to the crew that if they ever did that again, i would personally insert the GPU cable in a place where the sun dont shine.
Never happened again.