Well, one "note" was to complain about the fan last week, and the second note was thanking us for the apple bread which we included with our reply. Our reply was also by note, since they weren't home when I went over. They came over yesterday afternoon while I was at work, noise ordinance in hand, and my wife was upset by their visit.
I went over last night, and they seem like perfectly nice, normal people. The noise drops off sharply to either side of their deck -- a little into their yard or around to the side, it's nearly inaudible. While I still find it relatively quiet, I suppose I can see how a constant sound could be irritating. As to legal limits, the problem is that there's not a dBA level stated in that clause of the noise ordinance -- it's "unreasonably interferes with ... peace, comfort and repose." Hence the original question about what constitutes "unreasonable." There is a simple dBA limit defined for unincorporated areas, which we aren't, but it's a useful reference standard. Cell phone microphones vary widely -- his cell phone says it's well over the limit for daytime noise, mine says it's slightly over the limit for nighttime noise. I don't know that I want to drop the money on a discrete sound measurement device so to support being a jerk and telling them to go away. Either way, if that were the applicable law, we would probably be at least a little too loud if anyone's cell phone is correct.
The trouble with saying "Is it louder than the HVAC?" is that no one has A/C. In this area, you find central air mostly in houses that are $700k+; at our less-extravagant level, neither they nor we have A/C. A few folks have window units, which we've also considered, but they're hard to find during a warm summer -- Home Depot gets a shipment and sells out the same day. Which also twists the argument that an attic fan won't save on the A/C cost -- we have no A/C cost we're trying to save on, because our house is cooled by whatever cooling effect the attic fan has, fans in the house, and open windows.
Now, if there's an argument to be made that attic fans don't actually cool the house, that's certainly one valid route. At this point, we've got a contractor coming over to see if there's something obviously wrong (rusted motor, bad bearings, etc.) and why the control switch doesn't turn it off. In the meantime, we've turned the thermostat up to 100 degrees (in the attic, not outside), which should substantially reduce how often it runs. Don't know whether we'll ultimately replace, remove, or just disable the fan. I'd rather not ignore them flat out -- my hope is that, even if they got the relationship off to a poor start, they might get to know us on a better level than "those people with the fan."
I'm not a financial advisor, I just play one on the Internet.