It's hard to explain - kind of a deep MMMMWWWWAAAAAHHHHHHH..... sound. Does that help?

Seriously, I would call it a muffled throaty sound. I am curious if the exhaust note of an engine that uses the Atkinson Cycle tends to sound different than the conventional Otto Cycle engines. My understanding is that the intake valve stays open longer, allowing some of the intake charge to reverse flow back into the intake manifold. It seems like this could change the sound of the exhaust, but clearly this is just my own speculation!