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: 
Hope this helps some of you, what solved my issue was the following,

* Open Terminal
* type `cd /Library/Preferences/Audio` and press Enter type
* `ls` and press Enter you should see two files:

```
com.apple.audio.DeviceSettings.plist
com.apple.audio.SystemSettings.plist

```

* type `sudo rm com.apple.audio.DeviceSettings.plist` and press Enter it will prompt you for your password
* type `sudo rm com.apple.audio.SystemSettings.plist` and press Enter this will delete the audio settings for your OS
* type `sudo killall coreaudiod` and press Enter that will restart the audio system and all should be working now, at least, it did for me.
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