i've the exactly same problem, the problems is when i plug-in the screen there's no VGA output neither on the DVI or HDMI. it also makes a sound, a bip and searching on the internet i've found that that sound it's because i'ts not pluged-in a video card.
i've pluged-in a geforce 9500 GT only to see if it was that and it worked perfectly, but i don't wanna use that video card. when i enter the BIOS and select the video source there's only PCI-E or PCI, no onboard card.
sorry for my English