Just bought a 2011 Expedition. After a day or two of owning it I realized it does this half honk about every 2 to 3 minutes with it off and key out. It's not a full horn blast, about 50% power. It does it sometimes when I close the passenger door and have heard it a few times while driving. I've done a lot of research: Does not have remote start, does not have aftermarket alarm. It seems those are likely causes, but I don't have them. Thanks in advance!

 

Not even sure if it's a honk. Noise comes from the rear. Any help out there?