I don't use my headphones very often. They're ancient but very good. I've been working at night on a client site with a flat above and I didn't want to disturb the residents.
I was playing music using the iTunes player on my Macbook Pro. I use a Teac HA-P50 DAC conected with an optical cable as my headphones are impossible for the Macbook to drive effectively (they have a 600ohm impedance).
I was getting audible distortion on most pieces I was playing. They were from CDs I'd ripped using iTunes lossless conversion.
After a lot of playing around, I swapped the optical connection and fed the DAC using a USB output. This improved things noticeably but I was still getting distortion.
I thought it might be my headphones finally dying, as they're neary thirty years old, but turning the volume down really low, the distortion was still audible in exactly the same bits of music.
I then tried new source material and bought a 24bit 48kHz FLAC remastered version of one of the CDs I already had. I couldn't play this with iTunes so I downloaded TEAC's high resolution audio player app. The disortion completely vanished. Not a hint of it. I then played the standard CD tracks which had caused problems earlier, and they had no distortion either.
It's the iTunes app itself (220.127.116.11 - the latest version on El Capitan) that actually causes the distortion.