It could be two things. THe first and easiest to check for could be a stuck key on the keyboard. Either the laptop keyboard itself, or a wired or wireless keyboard. Check all the keys for movement and travel, make sure none of them have anything sitting onthem even slightly, if a key won't move or is stuck, or has something like a crumb jammed underneith it at POST, it can make that repetitive beeping until the motherboard keyboard bios disables that key. Once booted, if any of your keys don't appear to work, thats the one.
The other thing it could be, and possibly more likely given when and how the beep is occuring, is a power on self test (POST) beep error code, possibly indicating a failing ram module. power supply module, or system board problem. That beep code can indicate any of those things, or just a stuck key.
If its power supply, that could be the voltage regulator module on the motherboard itself, the power brick or the battery. try booting on just battery or just power brick to eliminate one or the other of those. Ram, you'll want to download a memtest ISO file and burn it to a disk using something like nero and then run memtest. Or just buy upgraded ram since even laptop ram is cheap enough these days and swap it out.
If it isn't a stuck key, power supply, and it isn't the ram, its the motherboard or the voltage regulator on the motherboard. You will require a new one of those, which depending on your technical skill level and part availabilty, may mean a new laptop.