Alarm sound on the ideal €750 gaming PC (gigabyte board)

I built a new PC and based it on the $750 Gaming PC (Intel) recommendation.
https://www.computerbase.de/forum/t…g-pc-spiele-pc-self-compile.215394/

Intel Core i3-12100F, boxed with cooler
GIGABYTE B660M DS3H DDR4
Crucial Ballistix 16GB DIMM Kit
GC 500W Season Core
KIOXIA EXCERIA G2 1TB, M.2
https://geizhals.de/xfx-speedster-s…ore-gaming-rx-66xl8lfdq-a2619750.html?hloc=de
I use my old Thermaltake Swing RS 100 for the case.
New box fan:
https://geizhals.de/arctic-p12-pwm-pst-rgb-0db-schwarz-acfan00186a-a2508966.html?hloc=de

After assembling, it starts up, beeps (means everything is ok), and enters BIOS. Unfortunately, as soon as I can see the BIOS on the monitor, a kind of high-pitched signal tone starts, like a smoke detector, only lower. The noise is permanent and sounds like a kind of alarm tone.

So far I only know the beep codes from the motherboard, but since there is only 1 beep here, I thought everything was fine.
So far Google has told me it could be a temperature issue or fans not spinning.
All existing fans rotate. Only one box fan is installed.
The temperatures in the bios look fine.
I’ve already reset the BIOS to default (load optimal settings or something in Gigabyte).

I also noticed in the bio that

1) Low voltage at +12V, the value always stays below 12V (see picture), is it normal?
I connected the 2x4pin.

2) Under PCIE I see 4.0 x8 @ 4.0 x8
Shouldn’t I say x16?
(I put the M.2 in the top slot under the cooler).

After a short time, I also see an orange line between the motherboard and the graphics card.

I am so confused now by the alarm beeping that I don’t know how to proceed.
Bios is still running on Ver. P3, I wanted to get rid of the alarm before flashing.

I hope you can give me some advice