92 mustang LX started beeping when started

[amfw]

Help! It beeps like a microwave that's done----5 short beeps, a pause then again 5 times total-20 beeps, then it stops and won't do it again until I start it up again....plus, it's intermittent, doesn't do it all the time....I've had it for 7 years and it's never done it before----I just passed 86,000 miles....Anyone know what it is? It sounds like it's coming from under dash near radio and glove box!!!!!!

thanks, AMFW