Laptop flashing and beeping non stop

hmmm... < MacBook Air, macOS 15.5 >

if this is an Intel machine you can try—

resetting the  System Management Controller  Reset the SMC of your Mac - Apple Support 

resetting NVRAM/PRAM Reset NVRAM on your Mac - Apple Support

If this is an M-series mac—

A SafeBoot Start up your Mac in safe mode - Apple Support will sort many anomalies

Just to add to the previous suggestions 

Since you’ are running macOS 15.5 and the issue only started in the last few days, there are several possible explanations. 

These include a stuck or unresponsive keyboard key, a recurring system alert or notification, an accessibility setting, a software issue, or a hardware fault. 

The fact that pressing the Return (Enter) key temporarily stops the beeping is a useful clue, although it does not establish the underlying cause.

I recommend trying the following steps in order:

1. Check the keyboard and disconnect accessories

Check that the Return key and other keys are not stuck or obstructed. 

Disconnect any external keyboard, USB devices, hubs, or other accessories, then see whether the problem continues. If possible, note whether the issue occurs when you are not touching the keyboard.

2. Restart your MacBook Air

Save your work, choose the Apple menu  > Restart, and see whether the problem returns. If it does, note whether it begins immediately during startup, at the login screen, or only after you sign in.

3. Check notifications and accessibility settings

Open System Settings and review the Notifications and Accessibility sections. In particular, check whether VoiceOver, Voice Control, or other keyboard-related accessibility features are enabled. You can also try pressing Command-F5 to check whether VoiceOver is active; this shortcut toggles VoiceOver on or off.

Apple’s guides may help:

Change Accessibility settings on Mac

Turn VoiceOver on or off on Mac

Also, try to establish whether the display itself is flashing, the keyboard backlight is blinking, or an indicator light is flashing. These are different symptoms and may point to different causes.

4. Check for macOS updates

Go to Apple menu  > System Settings > General > Software Update and check whether an update is available for your Mac. Back up important files before installing updates.

Apple’s instructions are available here: Update macOS on Mac.

5. Run Apple Diagnostics if the problem persists

Apple Diagnostics can help identify certain hardware problems. The procedure depends on your MacBook Air’s processor:

• Apple silicon (M-series): Shut down your Mac. Press and hold the power button until the startup options appear. Release the button, then press and hold Command-D to start diagnostics.

• Intel processor: Turn on your Mac and immediately press and hold the D key until the diagnostic utility appears. If that doesn’t work, try Option-D.

Follow the onscreen instructions and record any reference codes displayed.

For detailed instructions, see How to use Apple Diagnostics to test your Mac.

When to seek assistance

If the beeping and flashing continue, occur before you sign in, or prevent you from using the Mac normally, I recommend contacting Apple Support or visiting an Apple Authorised Service Provider. If the Mac becomes unusually hot, smells burnt, or shows signs of battery swelling, stop using it and arrange professional assistance.

You can find support and service options here: Apple Support.

To help narrow down the cause, could you clarify two things?

1. What exactly is flashing? Is it the entire screen, the screen going black and returning, the keyboard backlight, or a particular indicator light?
2. What does the sound resemble? Is it a regular electronic beep, a repeated alert tone, or a sound that occurs when a key is pressed? Does pressing Return stop it completely, or only temporarily?

With those details, we should be able to narrow down the likely cause and identify the most appropriate next step without disrupting your work any more than necessary.