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.
Unplug from power outlet, "problem solved". Noise stopped, although this is a main issue from the power connector from the mac.

Instead of unplugging you can also plug into a grounded outlet using the appropriate extension cord. [This previous question](https://apple.stackexchange.com/q/17575/24324) has some useful info on that.