this might sound like a daft idea but!!!!!!!!
switch off PC and remove everything connected to the motherboard i.e. RAM, VGA card, all USB devices, monitor. all drives… keep PSU connected
switch it back on. If the same beep sound is present, you will know then the problem is either motherboard or PSU (more likely the motherboard). If the beeping has stopped switch off again and reconnect one thing at a time then switching back on each time you reconnect something to see when or if beeping returns
Try the following if you pc is beeping with everything but psu diconnected:-
Switch off the power again Check the battery voltage for around 3 volts or min 2.8 volts and if you think you can do it remove the CPU chip and carefully rub the pin side of the chip lightly on a metal surface, this will help dispel any static buid up inside the chip then reseat the CPU.
If the beeping is not present when you switch back on then bingo otherwise you might have a motherboard/CPU fault