Hum on Audio Now

[zamboknee]

Noticed there's now a hum when I plug in a 1/8" audio cable on my MBP. The cable is connected to a soundboard in a radio studio.
When I plug in my headphones (Beyerdynamic Custom Pro) I can't hear the hum.
My audio engineer seems to think it's the AC plug as it's much more pronounced when I plug the laptop into the AC adapter.
Any ideas on where this hum came from? How to fix it?

I'm going to plug it into a different AC Adapter but I can't do that until tomorrow morning so wanted to get some other thoughts in the meantime.
Thanks!

2015 MBP, High Sierra

 

[BrianBaughn]

If there's nothing wrong with your equipment my first guess would be a mains hum via a ground loop. You could try using a ground-lift adapter on your MBP power plug for this scenario.