There's only three things I have ever known to cause abnormal beeping sounds at startup.
Video
RAM
Stuck key
If it where a CPU/Mother board/PWR Supply problem, most likely it wouldn't beep at all and you would get no video.
One way or another, signs will most likely point to one of these three. Each of the systems I have worked on in the past had a code that would tell you exactly what the problems is. For example, with Compaq/HP systems you'll hear one long, and two short beeps acompainied by a flashing caps lock light to signify that it couldn't connect to the video contoller. Even on the early IBM systems, you would hear eight long beaps then a long pause for video and three short beeps for RAM. If it didn't beep at all, it would be CPU, motherboard, or power supply.
Do the beeps sound structured? If it does, check your video card and RAM. Start by trying to reseat these boards and cleaning the slots with compressed air.
If it is a contantant, repetative sound, try unplugging the keyboard, then power on. The beep should then go away and you should get a keyboard error, but at least you'll know what the problem is.
Good Luck!