I hooked up a Phase Perfect 15HP unit, and I was surprised by how loud it was, and the clunking. I'm not sure what the clunking is (Manual has no information but I found a Youtube video that has the same clunking) As a transformer, I should have know there would be noise, but this is pretty loud. Thinking about moving the unit outdoors. For anyone else considering a unit, if you like a quiet shop when your not using a power tool, you will want to plan on it being mounted externally.