Mystery Noise

Every morning at about 8am, there is this mystery noise at our house. As far as I can tell, it's coming from our pump house (we are hooked up to city water, but also have a well for irrigation). It makes this loud, grinding noise three times, and then stops. I have no idea what it is, and I also can't ever seem to arrange for J to hear the noise--on weekdays, he's already at work by 8am, and on weekends, he usually isn't up that early...or if I DO wake him up, of course, it doesn't make the noise. Figures!