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?
in notifications you have the option to make the sound but not show any message.... so go to notifications and see what you have selected to not pop up... i guarantee you it has "don't show anything" selected.