Question: Recently I got a quite nice pair of in-ear headphones which block noise very well (with a very tight seal, not noise cancellation). Before that, I used Apple earbuds.

With these better headphones, I'm now often hearing a hissing noise. This happens constantly while a sound is being played (music, alerts, etc.) and for several seconds after. I assume this was happening with my previous headphones but there wasn't a good enough seal for me to hear it.

This doesn't happen with my iPod or iPad. How can I stop it from happening?

Answer: 
I had the same buzz noise as questioned. Though, it went away when i unplugged my MacBook from the adapter. The buzz appeared every time i used a specific power socket. Changing the power socket made the noise go away.
I actualy heard the hiss when music isn’t on using earphones. i have beats pro heaphones. i found that when i disconnect power mag safe from macbook the hiss disapears completely. i guess it is electrical interference. also hear interference when i switch off heater when magsafe is connected. :D