4 Wheel Drive/SUVs: Loud noise while car NOT running, ford expedition, loud noise

Question
All of a sudden, my 1997 Ford Expedition started making this horribly loud noise from under the hood, first continuously, then intermittently with all the lights flashing in tune, with NO key in the ignition. When I tried to start it, it did not make any difference and kept doing it after I took the key out as well. Finally, it seemed to slow and the sound died after about 10 minutes. Now it just makes this loud click about every 5-10 seconds (again, no key in ignition). What might be wrong?

Answer
If thie truck has air assist suspention it may be the air compressor running when not needed. Ot os only supposed to run with the key on but wires can cross and switches can go bad so there may be an issue there. I have little knowledge of the system but thas what it sounds like to me

Matt