I have the 16gb black AT&T iPhone 4s and lately it's been making the tri-tone noise at random but shows nothing. That specific noise is only associated with the receipt of voice mails, but there are none when it makes the noise. I went into the reminder part to make sure it wasn't set and I verified that I want the phone to only make a noise once when it receives something (messages, tweets, etc). Anybody else having this problem?