Metronome sound for disk encryption password prompt
The older Android versions were using Full Disk Encryption (FDE) mode, therefore the password enter dialog was a small mini-OS booted before the actual Android-OS. 
This mini-OS was playing what you are a calling the "metronome sound" when it was waiting for the user password.

I assume this sound was introduced to ensure the user recognizes that the boot process requires user interaction. Because while the mini-OS is waiting for input the whole system is offline and no app is running - including no mobile voice/data or Wifi.

Nowadays Android use File Based Encryption (FBE) which means that every app can specify if a data is encrypted using the system key or the key that requires the user to unlock the device first. Therefore in theory if all apps are correctly implemented a large part of the system should already work before you enter your user password the first time after a reboot.

May be you are able to simulate the old behavior by installing an app that runs only after the Android device has booted up (BOOT_COMPLETED) and then plays a sound similar to your old system. Unfortunately I don't know a suitable app, but I assume programmable scripting apps like "Tasker" may allow you to create a script that plays a sound after the device has booted up.