same thing happened to me! Look under both front seats and something is unpluged! Plug it back in and it will fix your problem!!

 

just checked all plugs ...still beeping...now I have no power at my 12v socket but all fuses are good... I hope its not the computer

 

My 2001 F150 XLT does the same thing. It's at a dealer today for recall on cruise control (05S28) and I asked them what this beeping is about. Said it's secondary to the airbag indicator light on the cluster being burned out. Replace instrument cluster! Guess I'll live with the freaking beeping!

 

Five beeps in five times is the ABS malfunction indicator beep. My explorer does it because the LED in the dash is cut out (previous owners fix)