Try unplugging the keyboard while it’s off, the power it on see if it beeps.
Make sure all your fans are running (Ex. Power, CPU, Chassis) if they are set to trigger an alarm that would cause it.
If the fans are running, try temporially disabling all the alarms in the bios. Usually it is a constant beep if this happens.
3a. If that fixes it turn them on one by one. See which one is triggering it
If everything is working properly, and the fan speeds/temps are okay. I’d be out of suggestions without looking at it. Unplug the internal speaker (Unless it’s soldered on.)