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 know this is an old thread, but many still experience this issue and this is the forum that Google detects as a "top post" on the topic. 

I have some additional information to add, regarding the "unplug your power adapter" and "it is a grounding issue" responses.

1. Yes, unplugging the power fixes the problem, if you indeed have the "hiss" associated with the MacBook power supply.
2. It is not entirely a grounding issue. The MacBook "MagSafe" power supply is a complicated transformer with extensive digital circuitry. It creates noise.

Even if you bought a brand new, $79 MagSafe power supply, the noise would not completely go away. It may diminish significantly, even to the point that you cannot detect it, but it is never going away. It is an unfortunate anomaly of the Apple product lineup, and one that professionals in the recording industry circumvent by simply not using MacBook Pro models when clean-signal listening is mission-critical.

Sorry to be the bearer of the bad news. There is something to be said for improving grounding treatment, though. You'll notice the hissing decreases when you press one hand on the aluminum body of the MacBook Pro, and even moreso when pressing both hands on the MacBook. There are a lot of workarounds for improving the grounding on a MacBook Pro which diminish the hiss drastically. Good luck!