Buzz signal from back seat when put in drive.

I drove my van today and noticed when I shifted into drive, I herd like a warning signal from the back seat. I looked on my dash to see if any doors were open, they were not. No warning lights were on. I put it into park and the noise stopped. So I went to drive again and it started the noise again. I drove to the store and back with the same noise and no warning lights ever showed on the dash.