Do you have an electric fuel pump? Can you hear it during the glow plug cycle?
Mine noticeably changes pitch when the glow plug relay kicks in and out. If you get the glow plugs to light off by force, that is manually routing battery to the plugs by whatever means, make a mental note of how much the pitch varies. That may help you in knowing if your battery and plug circuit are normal.
Note I have a noisy Carter vane pump. I don't know if other pumps make as much whine as mine does. It's done this for years. It was my prime evidence of glow plug relay contact failure when the main contacts failed to make, and I was getting a click, but no juice. The pitch didn't change like it had done in the past.
Dead giveaway the plugs weren't getting juiced.