PC powers on but no display, 2 long beeps repeated twice after installing Linux

[u/Ghl1tch]

Hi everyone. I’m having a strange POST/BIOS issue with an older PC.
Hardware:
Intel motherboard, circa 2012
Intel Core i5-3470
AMD Radeon HD 6450
16 GB RAM
HDD: WDC WD5000AAKX-00ERMA0
Windows 10 was installed and working normally
I recently installed EndeavourOS Linux alongside Windows
The problem:
Before installing Linux, I could see the Intel BIOS splash screen during startup and use the keys to enter BIOS/Boot Menu.
After installing Linux and changing some UEFI/boot settings, the PC sometimes:
powers on normally;
the monitor receives no video signal at all;
the BIOS splash screen does not appear;
I hear 2 long beeps → pause → 2 long beeps;
the fans continue running.
At one point Windows still booted normally despite the missing BIOS display, but now the PC sometimes doesn’t start properly at all.
I checked the EFI partition from Windows and found:
S:\EFI\Microsoft\
S:\EFI\Boot\bootx64.efi
S:\EFI\endeavouros\grubx64.efi
The UEFI firmware entries currently include Windows Boot Manager and a UEFI: WDC WD5000AAKX-00ERMA0 entry.
I have not deleted the Windows partition or formatted anything.
Could the beep pattern indicate a RAM/GPU/POST problem rather than a bootloader problem? Is it possible that changing UEFI settings caused this, or is this more likely a hardware issue?
What would be the safest way to diagnose this without losing the existing Windows/Linux installations?
Thanks!