Mystery vibration on iPhone 4, please help!

I got my iPhone 4 one month ago.

Last Wednesday, I was on the Internet on my phone when it suddenly vibrated. 5 minutes later while I was sending a text, it did again.

I had no notifications.

It happened again once on Saturday. Again on Tuesday. And just now, twice in a row. It's never at the same time. It was at 8:39 today.

I have no notifications, still!

I've read that gmail can do this, but I can verify that it is not gmail because I have no new emails. I've also checked my notification settings and there's nothing that is only set on "sounds" to where I wouldn't get a push or badge.

I'm a little worried that something is wrong, because its becoming a recurring problem and I have no clue what it is.

Any ideas, kind iPhone experts?

Thanks,
CR