Q: Five beeps, no warning lights, 1989 Chevy S10

I drive the truck to work about 15 miles each direction. There are big hills each direction (long, steep, and kind of winding). The truck is in need of a valve adjustment (which will be done on the 7th by a YourMechanic.com mechanic). Other than that I didn't believe it was in need of anything else. About 13 miles into my trip home yesterday, the truck wasn't doing anything out of the ordinary. But I heard it beep 5 times in succession. No special pattern, no loop. Just 5 higher-pitched beeps at about 90bpm. It did it twice, about 5-8 minutes apart. I can't tell where it's coming from, my stereo has a receiver that hasn't worked in a while. The speakers work but doesn't sound like it's coming from them. It's weird, I can't tell where the beeping is coming from, I don't know what it means, and no warning lights pop up. There are no odd smells or sounds. It did it again this morning as I was pulling in to work, twice. About 30 seconds between sets. Google search was futile. Any ideas?

My car has 181470 miles.
My car has an automatic transmission.