Through a lengthy I finally found this: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1654448
Which hinted to the fact that "Heaphone Mic Boost" is to blame for this. Issuing: $ amixer -c 0 set 'Headphone Mic Boost',0 1 makes the problem totally disappear for me. But why it depends on which kernel has being booted? Hope this helps somebody out there; I found this problem very obnoxious.
