It could be the shock sensor that is setting it off. Any sizeable vibration will set it off. Such as another vehicle with deep rumbling exhaust, planes going overhead, construction, etc. I finally had to disconnect the shock sensor on my alarm because I worked close to an airport and whenever the planes would go overhead, it would set the alarm off.

Richard

Had an 89 Isuzu Pickup
Had a 94 Mitsubishi Eclipse
84 Honda XL185S
Had an 89 Isuzu Trooper w/ 3.4 V6
01 Toyota Tundra SR5 V8 4Door Access Cab
http://community.webshots.com/user/rsayloriii