I started hearing what could be described as a low-note horn in the distance, like a P/U truck 1/4 mile away, and the DIC showed 'CHECK FUEL CAP'. It seems the two are related, and I'm guessing something in the EVAP test control system is leaking. I just can't determine the source, as the sound doesn't seem to have a source. Could it be a leaking gas cap seal, the purge valve, or ???