MacMini Intel fanblow after power is connected

Hi!
Since a couple of month, my first generation intel core duo mac mini behaves strange: Whenever I plug in the powercord, the fan starts blowing at full speed. Meanwhile the mini can't be started. After a couple of minutes the blowing ends. After that the mini can be powered and is working without any trouble - fan blowing only under heavy load. When I shut the mini down, everything is fine and the system can be started, but whenever I disconnect power, the mini is behaving like that again. Already have reset SMC - with no effort!

Thanx,
Marc