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: 
Thanks to Bart Arondson for tips to solving my own issue. I've posted details in case they're helpful to someone else.

I'm using a Heil PR40 Mic (Dynamic microphone) with a Mackie VLZ4 mixer, and was disappointed that (after spending a lot on equipment) I had an unacceptable hiss on my audio, regardless of output (headphones, speakers, etc.)

Following Bart's lead to the Audio MIDI Setup I changed the Input from the mixing board. I changed the Format to 24000 Hz, with 1ch-16bit Integer.

Frequencies below 24000 sounded hollow/aquarium-like, and frequencies above created the hiss/ringing that had been driving me crazy. Also, using the 2ch-16bit and the 1ch-8bit settings were unacceptable. Apologies if I mucked the link to the screenshot!

![screenshot of Audio MIDI settings] <https://www.dropbox.com/s/fmag17do6msqdyx/Screenshot%202014-09-03%2011.20.24.png?dl=0>
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!