Basically your laptop is running off the wall socket power whenever it is plugged in.
The presence of buzz in your audio says to me that there is not enough current coming from the wallwart for it to do a good job - this sort of buzz comes from devices that are OK for charging a device but not running it, I get the same problem when I run
my SH101 from a low current wallwart, but not a high current one (when it tries to pull the current it needs, it creates ripples on the dc lines which are the buzz that you can hear).
It is not a ground loop as most wallwarts are double insulated which means that there is no connection from it to ground in the first place. no connection to ground at the computer means no possible ground loop issues.
Best you can do is
- to use a couple of di boxes with ground lift switches to see if that fixes the problem or
- add some extra electrolytic capacitors to the power circuit to further reduce current ripple (not really a viable option unless you have the immediate skills) or
- get a high current wallwart for your laptop.