I notice a strange crackle while booting my new Dell Latitude E7470 with a fresh ubuntu on it. It happens twice on every boot. Once before entering the password and once after successful login.
I found a solution by toying around with the alsamixer.
For me the solution is to disable the loopback channel.
Start alsamixer with this command:
- Naviagate to the Loopback channel rightmost with arrow-key
- Disable the Channel by pressing the arrow-down-key once
- Save and quit the alsamixer with ESC
After that you need to reboot. The crackle is gone.