It May or May Not Be Hardware-Based
Some sources indicate that loop disease symptoms are caused by a software-related issue. You may want to try adjusting the Ring/Silent and volume settings on your device. A hard restart and resetting the settings (Settings > General > Reset > Reset All Settings) on your device could also help.
Unfortunately, loop disease can also reportedly be caused by a hardware issue. Specifically, one repair shop told Consumer Affairs that loop disease is a symptom of the audio chip on an iPhone’s motherboard coming loose. And this is the worst case scenario, since a hardware-related issue won’t be readily fixable by the end user.