For a new build, your thermal paste looks like it's seen years of service making me think your cpu is running very hot! What does cpu temp show in bios? Did you remove protective plastic film from the bottom of the cooler?
Try this:
Press Windows+Ctrl+T to make the current window always on top. A blue border will appear on the window, and a sound will play. Press Windows+Ctrl+T to undo your change and make Windows treat the window normally. You can customize this behavior by launching the PowerToys Settings window.
An easier way to test the switch is to use a multimeter set to ohms (resistance). Remove the switch's 2 prong header from the motherboard and place one probe to each pin of the plug. It should read OL indicating no conductivity. Now press and hold the power button switch. The meter should read 0...
That's an odd situation. I'm just guessing, but I don't think a cpu can cause that sound. I'd be tempted to say it's a component on the motherboard at that location doing it..perhaps related to powering like a voltage regulator say. All your psu cables are fully connected and snug? You might try...
Roll up a sheet of paper or get an empty paper towel tube, place it to your ear and try to isolate the sound that way. I'm tempted to think the psu is making that noise
Just a thought..if you swapped mobos and it's not the same as yhe original, and youre using the same Windows installation that ran the old board, then you could see issues that may confuse your results. As it is, it does sound like some power issue