If the computer is still usable while beeping it probably isn't RAM or motherboard failure as they generally cause crashes, lock-ups & failures to boot.
One possibility is the BIOS is configured to beep when the CPU reaches a certain temperature, warning things are getting a bit warm.
Check the fans on the CPU, in the power supply & on the GPU, plus any others you can find for dust & functionality. The CPU heatsink could be full of dust as well - a nice insulating blanket.
It's also possible the warning temperature is set low - settings can spontaneously change so check.