[Al Skierkiewicz]

I have two possible causes. Invoking the Nottke Rule (named after a past co-worker) go where the last person was. i.e. the Church. We had a similar problem in my neighborhood after the library changed their air handlers. The high volume (6-8 ft diamter) intake fan (vaneaxial fan is the term I think) they used required a pillow block secured to top, bottom and sides of the intake duct. The installers placed the block supports too close to the blades of the fan and the resulting turbulence as a blade passed by the supports resulted in a "hum" that exceeded daytime residential acoustic standards by several dB. The closest house faced the intake about 100 yards away. Turning off the fan was not enough to stop the blades as air passing through the intake kept the fan turning enough to create the noise. (for a library it was neccessary to have high volume, low velocity, air) The shutters on the intake had to be closed before the fan would stop. Since the turbulence was on the outside of the fan assy, most of the noise went right out the intake. There was a little noise in the library.
Second possible occured in a recording studio out west. The owners decided to place the studio way out in the country where there were no airports or factories and the closest road was more than a mile away. To top it all off, when the core samples came back they found some sizable bedrock under neath the planned studio location. So the owners thought they would have no problem. After all the money was in and construction was finished they found a serious low frequency noise problem that couldn't be explained. Consultants were called in and during one of the investigations, the consultant looked out the window and noticed a big truck driving on the road in the distance. When the truck was gone, so was the noise. However, the truck was hauling rock from a nearby quarry. It turns out that the "rock" they had built on turned out to be just that, a large rock. One end came up under the road and the other end came up under the studio. Push on one side and the other side moves. In between the rock was below grade so no one thought there was a connection. It turns out that sound travels faster through rock than through air.