Have u put LEDs in them ? If so its called "cold diagnostics" it's the cecm pulsing the circuits, its part of but not soly to do with the bulb out warning system, it's only cos it's LEDs it picks up the lower voltage sent to the light, if it were a normal bulb it would ignore this and you would never notice.
Lots of people who have rear LEDs get this also but don't see if often enough
