Some folks find that the iPhone’s noise-cancellation feature causes a weird, uncomfortable sensation. It’s a product of the way the technology works, as Apple Discussion member KiltedTim says, linking to HowStuffWorks:
“In order to cancel out background noise, the sound is not “eliminated” from the audio stream you’re hearing. It is countered by a second audio stream that basically eliminates your ability to hear it.
Since the sound and the “counter sound” are still hitting your eardrum, this can result in an odd sensation. Your ear is processing the sound, but your brain isn’t registering it. Since the inner ear controls balance, this will produce a dizzying effect in some people.”
If this is bothering you, here’s how to turn off the noise-cancellation feature, which Apple added to iOS 7. (Originally exclusive to the iPhone 5s, noise cancellation is now available to older devices in iOS 7.1.).