1994 F150 Beeping when brakes applied/flashing airbag light code 5 1

After driving three hours, we pulled off the interstate and when I applied the brakes, we got a beeping (alarm-type) noise that sounded like it was coming from under the dash or under the seat--it was hard to tell. The noise quit when I released the brake and was intermittent when I applied the brake after that; sometimes it beeped when I put the brakes on and sometimes it didn't. There were no lights associated with the brakes, the ABS or the airbag on the panel.

Ironically, we were pulling into a Ford dealership to buy an Expedition for my wife. I asked the service manager about the noise and he said it might be a brake servo module under the dash. I wasn't able to leave the pickup with them to troubleshoot or fix, so I headed home, about 40 miles.

On the way home, the beeping noise was gone, but the airbag light flashed a 5 1 code continuously.

Any suggestions about a cause for either the beep or the airbag light?
Is it something that I can replace, or should it be taken to a shop?

Thanks,
Ken

Here is a link to abs troubleshooting. I wish I knew more but hopefully this will help. Diagnosis and Testing (http://www1.autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/0a/a4/89/0900823d800aa489.jsp)

This link is for the air bag system.
Air Bag Diagnostic Monitor (http://www1.autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/0a/37/86/0900823d800a3786.jsp)
Use the previous and next arrows in the box for more info.
Good hunting.