My computer won't start after being turned off a couple of days ago. About 30 seconds after turning it on, I hear a "doo daa", "doo daa", "doo daa" coming from the unit. It sounds like an English police or ambulance. The monitor is on but stays black, and the pc won't turn off until the electrical current removed. I already changed the power supply and reseated the memory chips. I'm wondering if it is a m-b or cpu problem. I have an Intel P4 3.0 The m-b is MB-DFI PS35-BL P4-800/533 mhz. Thanks for any help you can provide.