fellow s-crewers,

ever since my truck was brand new i've always noticed what feels like a slight thunk within a few seconds after i fire up the engine. it is a sensation similar to that felt when the a/c compressor kicks in. however, my truck always does this, even with the climate controls on off. anyone know what that noise/vibration sensation is and what causes it, some kind of fan clutch or something? not really worried about it, just curious if anyone else has noticed similar...