Does the comp actually boot up? It could be the CPU overheating, At least AMDs (I think) have the habit of making constant noise when they overheat, make sure that the HSF is seated properly and the thermal paste is good.
The make and model as well as what is actually being heard when is the best thing to go by when trying to isolate the hardware itself. Since each different bios will not always point at the same thing that information is the first place to start. On most boards however a constant steady single beep is not good! unless something was left unplugged?