Alarm from back of Venture when in gear.

I have a 99 chevy venture. It has been a complete money pit. This morning while driving out of the blue an alarm began sounding from the rear of the van. Stopped and checked that all doors were shut since it sounded like the alarm for the doors. All doors were shut, but as soon as the van is put in gear the alarm goes off again. Drove the van about 20 miles to work with the alarm sounding all the way. Does anyone have any idea what the alarm could be for?

The heads were redone 2 weeks ago, as well as the t-stat, water pump, fan relays, upstream o2 sensor and a new vac line. I still need to install a new knock sensor as well as new coil packs, but I'm sure that can't be causing the problem. Could something in those repairs have caused the alarm, it has run fine for 2 weeks since the repairs. At this point I am stumped and tired of putting money into the van. Any help would be appreciated.

when the alarm sounds try pushing the electric side door disable button on the overhead console. if the alarm quiets then the switches for the side door need to be lubed as the top one has to push in as the door closes for the alarm not to sound. this is a common problem that is worse when it is cold.