If you are using the analog cable, leading to your CDROM drive,
you can disconnect that one and remove the CD analog cable
entirely. It may pick up electrical noise from inside the
computer. The computer can still play audio CDs, by means
of the ribbon cable, and digital transfer (DAE).


Do you think it could be this?

On your Mixer panel, in the recording section, mute any unused inputs.
Then try the Line_In test again.


I can set up only one input at a time.
I should try.
Thanks,
tano