I had the same thing...if you have Google Voice installed, and an active Google Voice Phone number, check to see if you are getting a call at that time. I have and I am. My GV number is forwarded to my wife/business partner while I am on client site, as it is the one I give out on my Resume, and I don't want to field calls from recruiters while on client site. It just happened to me, and sure enough, a call came in at that time. It is a strange "ring tone" like sound. I haven't investigated silencing it, but since there are no icons or popups, it is a rather useless notification on a Tablet. If I happen to have my BT headset connected to the tablet, I might actually be able to answer the call...I'll have to try that one of these days.