Sienna Beeping when car is in gear

Most likely you have a door switch that is going bad, so the van thinks a door is open (most likely a sliding door). You can verify this by looking at your dome lights—if they are coming on when it is beeping (if you have it set to turn on the lights when the doors are open). Here is an article on how to replace them, it is an easy repair: http://www.shareyourrepair.com/2013/02/how- to-fix-sienna-dome-lights-randomly.html