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 have had the same hissing with my MacBook Pro and a pair of Sennheiser CX300-II in-ear headphones. I solved this by changing the output format of the Built-in Output device from 48kHz/2ch-16bit to 48kHz/2ch-32bit.

This can be done as following:

* Open `Audio MIDI Setup`. Use Spotlight to find it or find the application in your Utilities folder
* Make sure your headphones are plugged in
* Click on `Built-in Output'
* On the right it will say `Source: Headphones` and format will be `48000.0 Hz / 2ch-16bit`
* Click on `2ch-16bit` and choose `2ch-32bit` from the drop down menu

I choose `2ch-32bit`, but I've noticed that any other value than `2ch-16bit` already eliminates the noise.

I am still in the process of figuring out why this works, anyone with more understanding of this issue, please feel free to edit my answer accordingly.
If the hissing sound goes away when on battery power, then you may a floating ground when you are plugged in. More info can be found at <https://superuser.com/questions/495517/electric-shock-when-touching-my-networked-installation-behind-ups/495521#495521>