Beeping noise after starting 03 GT

Here's the issue...after i start my 03 GT and have seat belt on there is a beeping noise within a minute after. It is 5 short beeps, 1 second or less in length and 1 second apart. It is 5 sets of 5 beeps then nothing for about 5 minutes. Then it does it again and continues every 5 minutes or so. It is not the seat belt warning; that makes a different beep and the seat belt light flashes. This sounds like the same beeping noise as when you have the keys in the ignition and the door open but the beeps are a little slower and in the pattern described above. Just wondering if anyone else has had this problem and what it might be. I think I located the source of the noise. The fuse box under the dash on the drivers side has a black box attached to the back closest to the firewall with wires/wiring harness coming from the top...this appears to be where the noise is coming from.

I am getting ready to leave for work and will check that out. i will let you know when i get to work.

air bag light didn't come on...something definetly going on with the airbag system.

ok, so I finally had time to check the bulb for the airbag and (of course) there wasn't one. So i took the bulb for the brake light out and put in the slot for the airbag. With everything back together the airbag light is on now....and so is the ABS light. The airbag light only flashed once after the car was started, then another time on the way to work. One flash only though. I assume the ABS light is on because i robbed the bulb for the brake?? Also, I thought the air bag light would blink the number of times to correspond to a code? Not sure what once flash is supposed to mean.

Forgot to mention...the beeping did stop once the bulb was installed.