Someone mentioned about this same ship's fog horn thing a while ago, but I thought mine sounded more like a deep buzz. Anyway, aside from the accuracy of the actual noise, I still think that its the vacuum circuit thats stuffed or one of the other smaller components along the circuit rather than the vacuum pump itself(hopefully!). Didn't you hear this noise during the test drive? I have seen people and car salesmen who deliberately turn on the car radio or show off the hifi loudly during the test drive to drown out any suspicious noises they know will warn you of problems ahead. This is what happenned to me. Are all the vacuum activated stuff working like the doors, aircon thingies(under the dash) and the dropping rear headrests(if you have them)??? Good luck in your search and report your progress, thanks.