Long beeping tone?

[alienmagic]

Today I was driving to work when all of a sudden I heard a tone (similar to what you hear on the radio or on tv when there's an emergency test), a short tone, followed by a longer one. I thought it was on the radio, but it was kind of loud, so I turned off the radio. The second, longer tone continued for about 4 seconds, then stopped. That was it, didn't happen again after that. Is this some warning feature that I don't know about?

[mjohn]

what kind of a truck is this? engine size?
And, more info. is needed...?