blank screen after normal boot up, external monitor works, fan noise

Hi,

my MacBookPro 15" Core 2 Duo (2,66) (running lion) started suddenly to show following behavior:

*boots up normally, screen shows apple logo and turning wheel, (also gets into target mode, boot disk selection mode etc normally (that means on the primary display)

*then, just before login window, the screen blanks. if an external monitor is attached, the login screen appears on the second screen. "About this mac" only shows the secondary display.

*pram reset, battery reset, same behavior. booting from lion#s recovery disk: same. booting from snow leopard external usb-disk: same.

Any ideas?

UPDATE:

i had noticed that the fans was doing a more noise than normal (sorry for not posting this information, thought it would be due to repeated booting). During one startup the noise went down, and i was able to boot normally... this lasted only a few minutes, during the next startup both problem (fan noise and display blackout) persisted. The noise seems to affect the left fan specifically. Computer is otherwise running normally, i am just writing this lines with it. Orderer a fan from ebay to day, lets see. Still, anybody knows an explanation?