Ok... exactly where is this beeping sound coming from?
Is it from the speaker or from some component?
If it's from the speaker... it might be any number of things, including a temperature warning... check everything make sure you've got the heatsink on right, CPU seated correctly in the socket, wiring connections solid etc. (I know you've already done some of this, but it can't hurt to do it again)
If it's not a deliberate warning, you've got a bad sub-assembly in there someplace... If the noise persists after being sure all your voltages, temperatures etc are good, isolate it to the noisey board and get it replaced under warranty.
---><font color=green>It ain't better if it don't work</font color=green><---