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.
**UNPLUG THE CHARGING CABLE FROM YOUR POWER PORT!**

This may sound crazy, but as another User (DOM) has kindly suggested, (and was downvoted for,) this is a Grounding Problem with the Mac itself; not your wires, not the sample rate, not the bit-depth. 
**This solved my (annoying) noise problem.** However, I think it may have to do with *which* outlet I plug my MacBook Pro *in* to.

I'm connecting my secondary MacBook Pro (mid-2012) into a Monster Power PRO 2500 Rack Power Center for my studio desk. It's plugged into the front outlet. I'm thinking that's causing my problem. However, it *may* be caused by *any* outlet I plug into since I've yet to test any others.

*Anyone else try using different outlets?*