What you are hearing is the warn-away feature of the multi-stage impact sensor. The alarm system is detecting impact. Motion should not do this, however a swift wind can. This can also be caused when there is creaking in the interior due to temperature changes.
Any alarm/stereo installation shop can adjust this for you. In a Viper, look under the driver's side dash. You should see a 1"x1" clear plastic box. This is the impact sensor. There is an adjustment screw in it. With a small screwdriver, left reduces sensitivity. With the system disarmed, there is still a red light in that impact sensor that illuminates when it triggers.
One good feature with Viper systems is the anti-nuisance mode. If the alarm is too sensitive, if triggered more than 2 times in 60 minutes, it silences the system. This makes sure a malfunctioning system doesn't cause too much trouble.