I thought mine was broken but clearly im not the only one... Great job again apple, great testing the product before you make a billion of them. DURRRRR...
Sound Problems on the iPhone 3G S?
"Then we heard about this. Boy Genius has reported that multiple users are experiencing sound issues with their new iPhone. Apparently right after a sound file is played the iPhone starts to play a continuous, high-frequency tone. They claim this tone is at around 15 Khz, which means it is too high for some people to even hear. For those who can, it is quite annoying.
If you're having this issue, setting your phone to vibrate will stop it for a while at least. It's probably a software issue, in which case we should see a fix to this come in the next couple of days. No other news sites have reported the issue, but I expect we'll hear more about it over the next day or two as the release hubhub dies down."
This is a real problem when recording video because the mic is right next to the speaker and my videos now have a hiss in them. It also makes this sound when you lock the phone... Go figure...