None of Seasonic X PSU's fans spin for regular wattage usage, they are dead silent. When I say dead silent there is more noise putting your ear next to a Rolex Oyster watch than a Seasonic X in regular desktop usage. I do have however some Silverstone FT02 cases that have excellent airflow, plus they have dedicated air-intake for the PSU. I expect if the Seasonic X was in a case with poor airflow then it's fan may come on sooner.
They do however start to spin once the wattage comes up, but its like a gentle spin just to get air moving. I had a X650 where the fan started to turn using a AMD R9 290 playing games, however the noise from the graphics cards, case, CPU fans was far above what the noise the X650 was producing even then.