As I said, dunno exactly what was wrong with the video card, but after replacing it the siren stopped...I could get it to stop by mucking with the display settings and changing the card's sync rate and/or resolution, but it only stayed quiet a couple of minutes and then it started again, and it is indeed much like a continuous two-tone siren, produced by the on-board bleeper of your mobo...yet the display didn't look as if the card was about to give in...does your card have an onboard fan? maybe it's kaput and the GPU is overheating?
ah...did a quick search...was indeed the fan on the GPU...and the siren is your mobo's overheating alarm, so it can also be that your CPU fan is on the fritz, check the hardware monitor page of your BIOS to see if the temp for mobo or CPU isn't running too hot...