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: 
Tough to know for certain, but my money is on an [impedance mismatch](http://en.wikipedia.org/wiki/Impedance_matching) between the output of the Macbook and your new headset. Or, perhaps more accurately stated, excessively sensitive (low impedance) headphones are revealing flaws in the MacBook audio output circuit.

Care to share the model of headphones you're now using? My guess is it's a low-impedance model intended for use with portable audio devices. You can boost the impedance by adding an in-line volume control adaptor like the Shure EA650. You will find other suggestions for solving this problem, including some DIY answers, by [searching the forums at Head-Fi.org](http://www.head-fi.org/search.php?search=increase%20impedence)

For more information/background see: [*Impedance, and how it affects audio equipment*](http://sound.westhost.com/impedanc.htm) by Rod Elliot and Sound On Sound: [*Understanding Impedance*](http://www.soundonsound.com/sos/jan03/articles/impedanceworkshop.asp).

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Another possible solution:
I have M-Audio powered monitors plugged into my MacBook Pro, and plug my Etymotic earplugs into the headphone jack on the monitors for private listening. I haven't noticed any noise problems with this set-up.
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.