why does my power brick go from green to yellow when I start my xbox 360 up?

The warranty is over and when I opened it up and turned it on, the fans start to spin for a split second and shut off and I look at my power brick and it turns from yellow (off) then to green back to yellow in a split second. Does anyone know of what could be the problem or could someone show me a website or forums that talk about xbox 360 problems or x clamping 360?