Regardless of the source of the interference (dirty mains power, the power supply, a faulty component, or something in the laptop), there is a strong link between the audio interface and the interference that ends up in the audio path. If the manufacturer cut corners in the shielding of the audio components, EMI-noise ends up in the audible range.
I've had an Edirol interface years ago, and I sold it because I could hear hard-disk activity, screen redraws and processor activity through the audio outputs.
This noise became a lot quieter when I disconnected the power supply from my laptop, but it was still there if I turned up the volume.
Other interfaces did not have this problem.
A transformer might help, but try to borrow one to see if it actually improves things, and while you're borrowing, also try out a decent audio interface to check what that can do for you.