When troubleshooting noise while using the iPhone 4S, the following article provides the most relevant information:
If this happens only when using the receiver, do the following:
- If you have a protective case on the phone, either ensure that the top microphone next to the headset jack is not covered or remove the protective case completely.
- If you have installed a protective film on the display, either ensure that the receiver is not covered or remove the film completely.
- Check the receiver mesh; if it appears blocked, use a clean, small, dry, soft-bristled brush to carefully and gently brush away any debris.
- If the audio issue happens only in one location, try another location.
- Try restarting the iPhone.
- If restarting the iPhone didn't resolve the issue, update to the latest version of iOS.
- If the issue persists, visit the iPhone Support Page for more troubleshooting and service information.